x86-64: Add vector atan/atanf implementation to libmvec

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

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