cmd/gc: turn race detector off for tail-call method wrapper functions

It was off in the old implementation (because there was no high-level
description of the function at all). Maybe some day the race detector
should be fixed to handle the wrapper and then enabled for it, but there's
no reason that has to be today.

R=golang-dev
TBR=dvyukov
CC=golang-dev
https://golang.org/cl/10037045

src/cmd/gc/go.h

@@ -268,6 +268,7 @@ struct	Node
 	uchar	dupok;	// duplicate definitions ok (for func)
 	schar	likely; // likeliness of if statement
 	uchar	hasbreak;	// has break statement
+	uchar	norace;	// disable race detector for this function
 	uint	esc;		// EscXXX
 	int	funcdepth;
 

src/cmd/gc/racewalk.c

@@ -58,7 +58,7 @@ racewalk(Node *fn)
 	Node *nodpc;
 	char s[1024];
 
-	if(ispkgin(omit_pkgs, nelem(omit_pkgs)))
+	if(fn->norace || ispkgin(omit_pkgs, nelem(omit_pkgs)))
 		return;
 
 	if(!ispkgin(noinst_pkgs, nelem(noinst_pkgs))) {

src/cmd/gc/subr.c

@@ -2574,6 +2574,8 @@ genwrapper(Type *rcvr, Type *method, Sym *newnam, int iface)
 	
 	// generate call
 	if(isptr[rcvr->etype] && isptr[methodrcvr->etype] && method->embedded && !isifacemethod(method->type)) {
+		// generate tail call: adjust pointer receiver and jump to embedded method.
+		fn->norace = 1; // something about this body makes the race detector unhappy.
 		// skip final .M
 		dot = dot->left;
 		if(!isptr[dotlist[0].field->type->etype])