x86-64: Add vector pow/powf to libmvec microbenchmark

Add vector pow/powf and input files to libmvec microbenchmark.

libmvec-pow-inputs:
  arg1:
    90% Normal random distribution
    range: (0.0, 256.0)
    mean: 0.0
    sigma: 32.0
    10% uniform random distribution in range (0.0, 256.0)
  arg2:
    90% Normal random distribution
    range: (-127.0, 127.0)
    mean: 0.0
    sigma: 16.0
    10% uniform random distribution in range (-127.0, 127.0)

libmvec-powf-inputs:
  arg1:
    90% Normal random distribution
    range: (0.0f, 100.0f)
    mean: 0.0f
    sigma: 16.0f
    10% uniform random distribution in range (0.0f, 100.0f)
  arg2:
    90% Normal random distribution
    range: (-10.0f, 10.0f)
    mean: 0.0f
    sigma: 8.0f
    10% uniform random distribution in range (-10.0f, 10.0f)

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