This CL introduces a new cpu package for CPU/platform feature detection.
The cpu package is basically a copy of `internal/cpu` of the standard library.
Revision: bf86aec25972f3a100c3aa58a6abcbcc35bdea49

This CL does not export ARM64 and PPC64 feature detection since at the moment
ARM64/PPC64 requires standard library support.

Updates golang/go#24843

Change-Id: I11bc1ca60b116e902c941b5887c00870dbb1f899
Reviewed-on: https://go-review.googlesource.com/107015Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>