Uses the same implementation as runtime/internal/atomic.

Reorganize the intrinsic detector to make it more table-driven.

Also works on amd64p32.

Change-Id: I7a5238951d6018d7d5d1bc01f339f6ee9282b2d0
Reviewed-on: https://go-review.googlesource.com/28076Reviewed-by: Cherry Zhang <cherryyz@google.com>