x86: Use default libc_fe* implementation for float128azanella/x86-libc_fe

The exported x86_64 fenv.h functions operate on both i387 and SSE (since
they should work on both float, double, and long double) while the
internal libc_fe* set either SSE (float, double, and float128) or
i387 (long double).

The rounding mode is used by soft-fp (libgcc and glibc), so that must be
set for float128 computations.  For x86_64 it uses SSE mode, while for
i386 is uses x87.

However for exception generation both 387 and SSE exception might
be generated even for x86_64.  The FP_EX_DIVZERO, FP_EX_DIVZERO, and
FP_EX_INEXACT uses SSE while FP_EX_DENORM, FP_EX_OVERFLOW, and
FP_EX_UNDERFLOW uses x87.

For i686 it depends whether libgcc was built for SSE math (where it
follows x86_64 behaviour) or x87 math (which uses only x87 mode).

In any case use generic routines to handle float128 since it handles
both modes.  It allows to use libc_fe* macros on float128 implementation
for x86_64.

Checked on x86_64-linux-gnu.

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