runtime: use FP offsets for pipe/pipe2 on freebsd/arm64 and linux/arm64

This is more readable and less error-prone than using RSP offsets.

Suggested during review of CL 212765.

Change-Id: I070190abeeac8eae5dbd414407602619d9d57422
Reviewed-on: https://go-review.googlesource.com/c/go/+/213577
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>

src/runtime/sys_freebsd_arm64.s

@@ -129,7 +129,7 @@ ok:
 
 // func pipe() (r, w int32, errno int32)
 TEXT runtime·pipe(SB),NOSPLIT|NOFRAME,$0-12
-	ADD	$8, RSP, R0
+	MOVD	$r+0(FP), R0
 	MOVW	$0, R1
 	MOVD	$SYS_pipe2, R8
 	SVC
@@ -141,7 +141,7 @@ ok:
 
 // func pipe2(flags int32) (r, w int32, errno int32)
 TEXT runtime·pipe2(SB),NOSPLIT|NOFRAME,$0-20
-	ADD	$16, RSP, R0
+	MOVD	$r+8(FP), R0
 	MOVW	flags+0(FP), R1
 	MOVD	$SYS_pipe2, R8
 	SVC

src/runtime/sys_linux_arm64.s

@@ -112,7 +112,7 @@ TEXT runtime·read(SB),NOSPLIT|NOFRAME,$0-28
 
 // func pipe() (r, w int32, errno int32)
 TEXT runtime·pipe(SB),NOSPLIT|NOFRAME,$0-12
-	ADD	$8, RSP, R0
+	MOVD	$r+0(FP), R0
 	MOVW	$0, R1
 	MOVW	$SYS_pipe2, R8
 	SVC
@@ -121,7 +121,7 @@ TEXT runtime·pipe(SB),NOSPLIT|NOFRAME,$0-12
 
 // func pipe2(flags int32) (r, w int32, errno int32)
 TEXT runtime·pipe2(SB),NOSPLIT|NOFRAME,$0-20
-	ADD	$16, RSP, R0
+	MOVD	$r+8(FP), R0
 	MOVW	flags+0(FP), R1
 	MOVW	$SYS_pipe2, R8
 	SVC