runtime: use clock_gettime for time.now() on s390x

This should improve the precision of time.now() from microseconds
to nanoseconds.

Also, modify runtime.nanotime to keep it consistent with cleanup
done to time.now.

Updates #11222 for s390x.

Change-Id: I27864115ea1fee7299360d9003cd3a8355f624d3
Reviewed-on: https://go-review.googlesource.com/27710Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/runtime/sys_linux_s390x.s

@@ -171,30 +171,26 @@ TEXT runtime·mincore(SB),NOSPLIT|NOFRAME,$0-28
 
 // func now() (sec int64, nsec int32)
 TEXT time·now(SB),NOSPLIT,$16
-	MOVD	$0(R15), R2
-	MOVD	$0, R3
-	MOVW	$SYS_gettimeofday, R1
-	SYSCALL
-	MOVD	0(R15), R2	// sec
-	MOVD	8(R15), R4	// usec
-	MOVD	$1000, R3
-	MULLD	R3, R4
+	MOVW	$0, R2 // CLOCK_REALTIME
+	MOVD	$tp-16(SP), R3
+	MOVW	$SYS_clock_gettime, R1
+	SYSCALL
+	LMG	tp-16(SP), R2, R3
+	// sec is in R2, nsec in R3
 	MOVD	R2, sec+0(FP)
-	MOVW	R4, nsec+8(FP)
+	MOVW	R3, nsec+8(FP)
 	RET
 
 TEXT runtime·nanotime(SB),NOSPLIT,$16
 	MOVW	$1, R2 // CLOCK_MONOTONIC
-	MOVD	$0(R15), R3
+	MOVD	$tp-16(SP), R3
 	MOVW	$SYS_clock_gettime, R1
 	SYSCALL
-	MOVD	0(R15), R2	// sec
-	MOVD	8(R15), R4	// nsec
-	// sec is in R2, nsec in R4
+	LMG	tp-16(SP), R2, R3
+	// sec is in R2, nsec in R3
 	// return nsec in R2
-	MOVD	$1000000000, R3
-	MULLD	R3, R2
-	ADD	R4, R2
+	MULLD	$1000000000, R2
+	ADD	R3, R2
 	MOVD	R2, ret+0(FP)
 	RET