cmd/internal/gc: sync nowritebarrier checks and write barrier insertion

Change-Id: I348223d0336e28d95b8e68d7653aa547acc7c9c3
Reviewed-on: https://go-review.googlesource.com/10262Reviewed-by: Austin Clements <austin@google.com>

src/cmd/internal/gc/cgen.go

@@ -785,6 +785,9 @@ abop: // asymmetric binary
 var sys_wbptr *Node
 
 func cgen_wbptr(n, res *Node) {
+	if Curfn != nil && Curfn.Func.Nowritebarrier {
+		Yyerror("write barrier prohibited")
+	}
 	if Debug_wb > 0 {
 		Warn("write barrier")
 	}
@@ -828,6 +831,9 @@ func cgen_wbptr(n, res *Node) {
 }
 
 func cgen_wbfat(n, res *Node) {
+	if Curfn != nil && Curfn.Func.Nowritebarrier {
+		Yyerror("write barrier prohibited")
+	}
 	if Debug_wb > 0 {
 		Warn("write barrier")
 	}

src/cmd/internal/gc/walk.go

@@ -2217,6 +2217,9 @@ func applywritebarrier(n *Node, init **NodeList) *Node {
 			return n
 		}
 		// Use slow path always for race detector.
+		if Curfn != nil && Curfn.Func.Nowritebarrier {
+			Yyerror("write barrier prohibited")
+		}
 		if Debug_wb > 0 {
 			Warnl(int(n.Lineno), "write barrier")
 		}