The implementation of atomics are inherently tricky. It would
be good to have them implemented in a single place, instead of
multiple copies.

Mostly a simple redirect.

On 386, some functions in sync/atomic have better implementations,
which are moved to runtime/internal/atomic.

On ARM, some functions in sync/atomic have better implementations.
They are dropped by this CL, but restored with an improved
version in a follow-up CL. On linux/arm, 64-bit CAS kernel helper
is dropped, as we're trying to move away from kernel helpers.

Fixes #23778.

Change-Id: Icb9e1039acc92adbb2a371c34baaf0b79551c3ea
Reviewed-on: https://go-review.googlesource.com/93637Reviewed-by: Austin Clements <austin@google.com>