runtime, misc/cgo/test: fix build for Linux/ARM

1. In CL 5989057, I made a mistake in the last minute change.
"MOVW.W R4, -4(SP)" should really be "MOVW.W R4, -4(R13)",
as 5l will rewrite offset for SP.
2. misc/cgo/test/issue1560.go tests for parallel sleep of 1s,
but on ARM, the deadline is frequently missed, so change sleep
time to 2s on ARM.

R=golang-dev, dave, rsc
CC=golang-dev
https://golang.org/cl/6202043

misc/cgo/test/issue1560.go

@@ -15,6 +15,7 @@ void twoSleep(int);
 import "C"
 
 import (
+	"runtime"
 	"testing"
 	"time"
 )
@@ -35,11 +36,17 @@ func BackgroundSleep(n int) {
 }
 
 func testParallelSleep(t *testing.T) {
+	sleepSec := 1
+	if runtime.GOARCH == "arm" {
+		// on ARM, the 1.3s deadline is frequently missed,
+		// so increase sleep time to 2s
+		sleepSec = 2
+	}
 	start := time.Now()
-	parallelSleep(1)
+	parallelSleep(sleepSec)
 	dt := time.Now().Sub(start)
-	// bug used to run sleeps in serial, producing a 2-second delay.
-	if dt >= 1300*time.Millisecond {
-		t.Fatalf("parallel 1-second sleeps slept for %f seconds", dt.Seconds())
+	// bug used to run sleeps in serial, producing a 2*sleepSec-second delay.
+	if dt >= time.Duration(sleepSec)*1300*time.Millisecond {
+		t.Fatalf("parallel %d-second sleeps slept for %f seconds", sleepSec, dt.Seconds())
 	}
 }

src/pkg/runtime/asm_arm.s

@@ -289,7 +289,7 @@ TEXT	runtime·cgocallback(SB),7,$16
 	// Save current m->g0->sched.sp on stack and then set it to SP.
 	MOVW	m_g0(m), R3
 	MOVW	(g_sched+gobuf_sp)(R3), R4
-	MOVW.W	R4, -4(SP)
+	MOVW.W	R4, -4(R13)
 	MOVW	R13, (g_sched+gobuf_sp)(R3)
 
 	// Switch to m->curg stack and call runtime.cgocallbackg