This fixes the following warnings:

In file included from /usr/include/stdio.h:894,
         from src/sbcenc.c:30:
In function ‘fprintf’,
  inlined from ‘encode’ at src/sbcenc.c:160:3,
  inlined from ‘main’ at src/sbcenc.c:329:3:
/usr/include/bits/stdio2.h:105:10: error: ‘srate’ may be
used uninitialized [-Werror=maybe-uninitialized]
 105 |  return __fprintf_chk (__stream, __USE_FORTIFY_LEVEL - 1, __fmt,
   |     ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 106 |             __va_arg_pack ());
   |             ~~~~~~~~~~~~~~~~~
src/sbcenc.c: In function ‘main’:
src/sbcenc.c:54:23: note: ‘srate’ was declared here
  54 |     int fd, size, srate, codesize, nframes;
   |            ^~~~~
cc1: all warnings being treated as errors

Don't call __builtin_cpu_init unless targeting i386 or x86_64.
Otherwise we get an error at link time:

  CC       sbc/sbc_primitives.lo
sbc/sbc_primitives.c: In function ‘sbc_init_primitives_x86’:
sbc/sbc_primitives.c:596:2: warning: implicit declaration of function ‘__builtin_cpu_init’; did you mean ‘__builtin_irint’? [-Wimplicit-function-declaration]
[...]
  CCLD     src/sbcdec
ld: sbc/.libs/libsbc-private.a(sbc_primitives.o): in function `sbc_init_primitives':
sbc_primitives.c:(.text+0x3a30): undefined reference to `__builtin_cpu_init'

Check if SBC_BUILD_WITH_MMX_SUPPORT is defined otherwise
sbc_init_primitives_mmx will not be declared and
_builtin_cpu_supports(“mmx”)) likely fail:

CC    sbc/sbc_primitives.lo
sbc/sbc_primitives.c: In function ‘sbc_init_primitives_x86’:
sbc/sbc_primitives.c:599:3: warning: implicit declaration of function
‘sbc_init_primitives_mmx’; did you mean ‘sbc_init_primitives_x86’?
[-Wimplicit-function-declaration]
 599 |  sbc_init_primitives_mmx(state);
   |  ^~~~~~~~~~~~~~~~~~~~~~~
   |  sbc_init_primitives_x86
sbc/sbc_primitives.c: In function ‘sbc_init_primitives’:
sbc/sbc_primitives.c:598:6: error: hwcap ‘mmx’ is an invalid argument to
builtin ‘__builtin_cpu_supports’
 598 | if (__builtin_cpu_supports(“mmx”))
   |   ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~
make[1]: *** [Makefile:711: sbc/sbc_primitives.lo] Error 1
make: *** [Makefile:453: all] Error 2

This adds a new API function sbc_reinit_msbc. Like sbc_reinit or
sbc_reinit_a2dp but for msbc.