cmd/internal/gc: fix race build

Add special case for OGETG which should never be instrumented.

Change-Id: I7d082abb8608537f82b03362b687baf2a1d809dc
Reviewed-on: https://go-review.googlesource.com/8551Reviewed-by: Dmitry Vyukov <dvyukov@google.com>

src/cmd/internal/gc/racewalk.go

@@ -410,6 +410,9 @@ func racewalknode(np **Node, init **NodeList, wr int, skip int) {
 		OLABEL:
 		goto ret
 
+	case OGETG:
+		Yyerror("racewalk: OGETG can happen only in runtime which we don't instrument")
+
 		// does not require instrumentation
 	case OPRINT, // don't bother instrumenting it
 		OPRINTN,     // don't bother instrumenting it