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

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

Add vector atan/atanf and input files to libmvec microbenchmark.

libmvec-atan-inputs:
  arg1:
    90% Normal random distribution
    range: (-DBL_MAX, DBL_MAX)
    mean: 0.0
    sigma: 4.0
    10% uniform random distribution in range (-1.0e6, 1.0e6)
  arg2:
    90% Normal random distribution
    range: (-DBL_MAX, DBL_MAX)
    mean: 0.0
    sigma: 4.0
    10% uniform random distribution in range (-1.0e6, 1.0e6)

libmvec-atanf-inputs:
  arg1:
    90% Normal random distribution
    range: (-FLT_MAX, FLT_MAX)
    mean: 0.0f
    sigma: 4.0f
    10% uniform random distribution in range (-1.0e6f, 1.0e6f)
  arg2:
    90% Normal random distribution
    range: (-FLT_MAX, FLT_MAX)
    mean: 0.0f
    sigma: 4.0f
    10% uniform random distribution in range (-1.0e6f, 1.0e6f)

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