PR go/84215
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.
Reviewed-on: https://go-review.googlesource.com/93197
From-SVN: r257599