runtime: Use old reflect.call implementation from cgo.

Basically a partial rollback of 12053043 until I can
figure out what is really going on.
Fixes bug 6051.

R=golang-dev
CC=golang-dev
https://golang.org/cl/12496043

misc/cgo/test/callback.go

@@ -151,7 +151,6 @@ func testCallbackCallers(t *testing.T) {
 	n := 0
 	name := []string{
 		"test.goCallback",
-		"runtime.call16",
 		"runtime.cgocallbackg1",
 		"runtime.cgocallbackg",
 		"runtime.cgocallback_gofunc",

src/pkg/runtime/cgocall.c

@@ -295,7 +295,7 @@ runtime·cgocallbackg1(void)
 
 	// Invoke callback.
 	cb = CBARGS;
-	reflect·call(cb->fn, cb->arg, cb->argsize);
+	runtime·newstackcall(cb->fn, cb->arg, cb->argsize);
 
 	if(raceenabled && !m->racecall)
 		runtime·racereleasemerge(&cgosync);