x86-64: Add vector expm1/expm1f implementation to libmvec

Implement vectorized expm1/expm1f containing SSE, AVX, AVX2 and
AVX512 versions for libmvec as per vector ABI.  It also contains
accuracy and ABI tests for vector expm1/expm1f with regenerated ulps.

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