Intrinsic enabled on all architectures,
runtime asm implementation removed on all architectures.

Fixes #21258

Change-Id: I2cb86d460b497c2f287a5b3df5c37fdb231c23a7
Reviewed-on: https://go-review.googlesource.com/53411Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>
Reviewed-by: David Chase <drchase@google.com>