cmd/gc: add //go:nowritebarrier to diagnose unintended write barriers

//go:nowritebarrier can only be used in package runtime.
It does not disable write barriers; it is an assertion, checked
by the compiler, that the following function needs no write
barriers.

Change-Id: Id7978b779b66dc1feea39ee6bda9fd4d80280b7c
Reviewed-on: https://go-review.googlesource.com/1224
Reviewed-by: Rick Hudson <rlh@golang.org>