Avoid defining f64xfmaf128 twice when building s_fmaf128.c.
This can be reproduced on powerpc64le whenever f128 functions do not
have IFUNC enabled, e.g. using "--with-cpu=power8 --disable-multi-arch", or
when using "-with-cpu=power9".
Fixes: b3f27d8150 ("Add narrowing fma functions")