[powerpc] fenv_libc.h: protect use of __builtin_cpu_supports

Using __builtin_cpu_supports() requires support in GCC and Glibc.
My recent patch to fenv_libc.h added an unprotected use of
__builtin_cpu_supports().  Compilation of Glibc itself will fail
with a sufficiently new GCC and sufficiently old Glibc:

../sysdeps/powerpc/fpu/fegetexcept.c: In function ‘__fegetexcept’:
../sysdeps/powerpc/fpu/fenv_libc.h:52:20: error: builtin ‘__builtin_cpu_supports’ needs GLIBC (2.23 and newer) that exports hardware capability bits [-Werror]

Reviewed-by: Florian Weimer <fweimer@redhat.com>
Fixes 3db85a98.