When -mapxf is used to build glibc, the resulting glibc will never run
on FMA4 machines. Exclude FMA4 IFUNC functions when -mapxf is used.
This requires GCC which defines __APX_F__ for -mapxf with commit:
1df56719bd8 x86: Define __APX_F__ for -mapxf
Reviewed-by: Sunil K Pandey <skpgkp2@gmail.com>