x86-64: Add vector atanh/atanhf implementation to libmvec

Implement vectorized atanh/atanhf containing SSE, AVX, AVX2 and
AVX512 versions for libmvec as per vector ABI.  It also contains
accuracy and ABI tests for vector atanh/atanhf with regenerated ulps.

Reviewed-by: H.J. Lu <hjl.tools@gmail.com>