libgfortran: Make sure glibc < 2.32 built powerpc64le-linux libgfortran doesn't use __*ieee128 APIs

I've just tried to build libgfortran on an old glibc system
(gcc112.fsffrance.org) and unfortunately we still have work to do:

[jakub@gcc2-power8 obj38]$ LD_PRELOAD=/home/jakub/gcc/obj38/powerpc64le-unknown-linux-gnu/libgfortran/.libs/libgfortran.so.5.0.0 /bin/true
[jakub@gcc2-power8 obj38]$ LD_BIND_NOW=1 LD_PRELOAD=/home/jakub/gcc/obj38/powerpc64le-unknown-linux-gnu/libgfortran/.libs/libgfortran.so.5.0.0 /bin/true
/bin/true: symbol lookup error: /home/jakub/gcc/obj38/powerpc64le-unknown-linux-gnu/libgfortran/.libs/libgfortran.so.5.0.0: undefined symbol: __atan2ieee128

While we do use some libquadmath APIs:
readelf -Wr /home/jakub/gcc/obj38/powerpc64le-unknown-linux-gnu/libgfortran/.libs/libgfortran.so.5.0.0 | grep QUADMATH
0000000000251268  000005e400000026 R_PPC64_ADDR64         0000000000000000 quadmath_snprintf@QUADMATH_1.0 + 0
0000000000251270  0000030600000026 R_PPC64_ADDR64         0000000000000000 strtoflt128@QUADMATH_1.0 + 0
00000000002502e0  0000011600000015 R_PPC64_JMP_SLOT       0000000000000000 ynq@QUADMATH_1.0 + 0
0000000000250390  0000016000000015 R_PPC64_JMP_SLOT       0000000000000000 sqrtq@QUADMATH_1.0 + 0
0000000000250508  000001fa00000015 R_PPC64_JMP_SLOT       0000000000000000 fmaq@QUADMATH_1.0 + 0
0000000000250530  0000021200000015 R_PPC64_JMP_SLOT       0000000000000000 fabsq@QUADMATH_1.0 + 0
0000000000250760  0000030600000015 R_PPC64_JMP_SLOT       0000000000000000 strtoflt128@QUADMATH_1.0 + 0
0000000000250990  000003df00000015 R_PPC64_JMP_SLOT       0000000000000000 cosq@QUADMATH_1.0 + 0
00000000002509f0  0000040a00000015 R_PPC64_JMP_SLOT       0000000000000000 expq@QUADMATH_1.0 + 0
0000000000250a88  0000045100000015 R_PPC64_JMP_SLOT       0000000000000000 erfcq@QUADMATH_1.0 + 0
0000000000250a98  0000045e00000015 R_PPC64_JMP_SLOT       0000000000000000 jnq@QUADMATH_1.0 + 0
0000000000250ac8  0000047e00000015 R_PPC64_JMP_SLOT       0000000000000000 sinq@QUADMATH_1.0 + 0
0000000000250e38  000005db00000015 R_PPC64_JMP_SLOT       0000000000000000 fmodq@QUADMATH_1.0 + 0
0000000000250e48  000005e000000015 R_PPC64_JMP_SLOT       0000000000000000 tanq@QUADMATH_1.0 + 0
0000000000250e58  000005e400000015 R_PPC64_JMP_SLOT       0000000000000000 quadmath_snprintf@QUADMATH_1.0 + 0
0000000000250f20  0000062900000015 R_PPC64_JMP_SLOT       0000000000000000 copysignq@QUADMATH_1.0 + 0
we don't do it consistently:
readelf -Wr /home/jakub/gcc/obj38/powerpc64le-unknown-linux-gnu/libgfortran/.libs/libgfortran.so.5.0.0 | grep ieee128
0000000000250310  0000012800000015 R_PPC64_JMP_SLOT       0000000000000000 __atan2ieee128 + 0
0000000000250340  0000014200000015 R_PPC64_JMP_SLOT       0000000000000000 __clogieee128 + 0
0000000000250438  000001a300000015 R_PPC64_JMP_SLOT       0000000000000000 __acoshieee128 + 0
00000000002504b8  000001cc00000015 R_PPC64_JMP_SLOT       0000000000000000 __csinieee128 + 0
0000000000250500  000001f300000015 R_PPC64_JMP_SLOT       0000000000000000 __sinhieee128 + 0
0000000000250570  0000022a00000015 R_PPC64_JMP_SLOT       0000000000000000 __asinieee128 + 0
0000000000250580  0000022d00000015 R_PPC64_JMP_SLOT       0000000000000000 __roundieee128 + 0
00000000002505a0  0000023e00000015 R_PPC64_JMP_SLOT       0000000000000000 __logieee128 + 0
00000000002505c8  0000024900000015 R_PPC64_JMP_SLOT       0000000000000000 __tanieee128 + 0
0000000000250630  0000027500000015 R_PPC64_JMP_SLOT       0000000000000000 __ccosieee128 + 0
0000000000250670  0000028a00000015 R_PPC64_JMP_SLOT       0000000000000000 __log10ieee128 + 0
00000000002506c8  000002bd00000015 R_PPC64_JMP_SLOT       0000000000000000 __cexpieee128 + 0
00000000002506d8  000002c800000015 R_PPC64_JMP_SLOT       0000000000000000 __coshieee128 + 0
00000000002509b0  000003ef00000015 R_PPC64_JMP_SLOT       0000000000000000 __truncieee128 + 0
0000000000250af8  000004a600000015 R_PPC64_JMP_SLOT       0000000000000000 __expieee128 + 0
0000000000250b50  000004c600000015 R_PPC64_JMP_SLOT       0000000000000000 __fmodieee128 + 0
0000000000250bb0  000004e700000015 R_PPC64_JMP_SLOT       0000000000000000 __tanhieee128 + 0
0000000000250c38  0000051300000015 R_PPC64_JMP_SLOT       0000000000000000 __acosieee128 + 0
0000000000250ce0  0000055400000015 R_PPC64_JMP_SLOT       0000000000000000 __sinieee128 + 0
0000000000250d60  0000057e00000015 R_PPC64_JMP_SLOT       0000000000000000 __atanieee128 + 0
0000000000250dd8  000005b100000015 R_PPC64_JMP_SLOT       0000000000000000 __sqrtieee128 + 0
0000000000250e98  0000060200000015 R_PPC64_JMP_SLOT       0000000000000000 __cosieee128 + 0
0000000000250eb0  0000060a00000015 R_PPC64_JMP_SLOT       0000000000000000 __atanhieee128 + 0
0000000000250ef0  0000062000000015 R_PPC64_JMP_SLOT       0000000000000000 __asinhieee128 + 0
0000000000250fd8  0000067f00000015 R_PPC64_JMP_SLOT       0000000000000000 __csqrtieee128 + 0
0000000000251038  000006ad00000015 R_PPC64_JMP_SLOT       0000000000000000 __cabsieee128 + 0
All these should for POWER_IEEE128 use atan2q@QUADMATH_1.0 etc.

It seems all these come from f951 compiled sources.
For user code, I think the agreement was if you want to use successfully
-mabi=ieeelongdouble, you need glibc 2.32 or later, which is why the Fortran
FE doesn't conditionalize on whether glibc 2.32 is available or not and just
emits __WHATEVERieee128 entrypoints.
But for Fortran compiled sources in libgfortran, we need to use
__WHATEVERieee128 only if glibc 2.32 or later and WHATEVERq (from
libquadmath) otherwise.

The following patch implements that, adds -fbuilding-libgfortran option
similar to e.g. -fbuilding-libgcc used when building libgcc and if
that option is set and the TARGET_GLIBC_{MAJOR,MINOR} macros indicate
no glibc or glibc older than 2.32, it will use the libquadmath APIs
rather than glibc 2.32 APIs.

2022-01-07  Jakub Jelinek  <jakub@redhat.com>

gcc/fortran/
	* trans-types.c (gfc_init_kinds): When setting abi_kind to 17, if not
	targetting glibc 2.32 or later and -fbuilding-libgfortran, set
	gfc_real16_is_float128 and c_float128 in gfc_real_kinds.
	(gfc_build_real_type): Don't set c_long_double if c_float128 is
	already set.
	* trans-intrinsic.c (builtin_decl_for_precision): Don't use
	long_double_built_in if gfc_real16_is_float128 and
	long_double_type_node == gfc_float128_type_node.
	* lang.opt (fbuilding-libgfortran): New undocumented option.
libgfortran/
	* Makefile.am (AM_FCFLAGS): Add -fbuilding-libgfortran after
	-fallow-leading-underscore.
	* Makefile.in: Regenerated.

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