x86-64: Add sysdeps/x86_64/fpu/Makeconfig

1. Add sysdeps/x86_64/fpu/Makeconfig to auto-generate libmvec.mk, which
contains libmvec ABI test dependencies and CFLAGS, in the build directory.
2. Include libmvec.mk for libmvec ABI test dependencies and CFLAGS.

Tested on SSE4, AVX, AVX2 and AVX512 machines.

Reviewed-by: Noah Goldstein <goldstein.w.n@gmail.com>