runtime: don't use ... formal argument to deferreturn.

R=golang-dev, rsc
CC=golang-dev
https://golang.org/cl/28860043

src/cmd/gc/pgen.c

@@ -168,8 +168,13 @@ compile(Node *fn)
 	if(retpc)
 		patch(retpc, pc);
 	ginit();
-	if(hasdefer)
+	if(hasdefer) {
 		ginscall(deferreturn, 0);
+		// deferreturn pretends to have one uintptr argument.
+		// Reserve space for it so stack scanner is happy.
+		if(maxarg < widthptr)
+			maxarg = widthptr;
+	}
 	if(curfn->exit)
 		genlist(curfn->exit);
 	gclean();

src/pkg/runtime/panic.c

@@ -157,14 +157,12 @@ runtime·deferproc(int32 siz, FuncVal *fn, ...)
 // is called again and again until there are no more deferred functions.
 // Cannot split the stack because we reuse the caller's frame to
 // call the deferred function.
-//
-// The ... in the prototype keeps the compiler from declaring
-// an argument frame size. deferreturn is a very special function,
-// and if the runtime ever asks for its frame size, that means
-// the traceback routines are probably broken.
+
+// The single argument isn't actually used - it just has its address
+// taken so it can be matched against pending defers.
 #pragma textflag NOSPLIT
 void
-runtime·deferreturn(uintptr arg0, ...)
+runtime·deferreturn(uintptr arg0)
 {
 	Defer *d;
 	byte *argp;