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