This patch to the Go frontend uses a 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 PR 84215; I'm not sure since I
haven't been able to recreate the problems described there.
Bootstrapped and ran Go testsuite on x86_64-pc-linux-gnu.  Committed
to mainline.

Ian