x86-64: Exclude FMA4 IFUNC functions for -mapxf

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>