runtime, sync/atomic: use write barrier for atomic pointer functions

This copies atomic_pointer.go from 1.10rc2.  It was omitted during the
transition of the runtime from C to Go, and I forgot about it.

This may help with https://gcc.gnu.org/PR84215.

Change-Id: If70fb6c827106142c7258a82c42c13b337d3e9ff
Reviewed-on: https://go-review.googlesource.com/93197
Reviewed-by: Than McIntosh <thanm@google.com>
3 files changed