runtime: fix stack pointer corruption in runtime.cgocallback_gofunc()

runtime.setmg() calls another function (cgo_save_gm), so it must save
LR onto stack.
Re-enabled TestCthread test in misc/cgo/test.

Fixes #4863.

R=golang-dev, r
CC=golang-dev
https://golang.org/cl/9019043

misc/cgo/test/cthread.go

@@ -8,7 +8,6 @@ package cgotest
 import "C"
 
 import (
-	"runtime"
 	"sync"
 	"testing"
 )
@@ -31,10 +30,6 @@ func Add(x int) {
 }
 
 func testCthread(t *testing.T) {
-	if runtime.GOARCH == "arm" {
-		t.Skip("testCthread disabled on arm")
-	}
-
 	sum.i = 0
 	C.doAdd(10, 6)
 

src/pkg/runtime/asm_arm.s

@@ -417,7 +417,7 @@ havem:
 	RET
 
 // void setmg(M*, G*); set m and g. for use by needm.
-TEXT runtime·setmg(SB), 7, $-4
+TEXT runtime·setmg(SB), 7, $0
 	MOVW	mm+0(FP), m
 	MOVW	gg+4(FP), g