runtime: fix use of STREX in various exitThread implementations

STREX does not permit using the same register for the value to store
and the place where the result is returned. Also the code was wrong
anyhow if the first store failed.

Fixes #22248

Change-Id: I96013497410058514ffcb771c76c86faa1ec559b
Reviewed-on: https://go-review.googlesource.com/70911
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Austin Clements <austin@google.com>

src/runtime/sys_darwin_arm.s

@@ -107,10 +107,10 @@ TEXT exit1<>(SB),NOSPLIT,$0
 TEXT runtime·exitThread(SB),NOSPLIT,$0-4
 	MOVW	wait+0(FP), R0
 	// We're done using the stack.
-	MOVW	$0, R1
+	MOVW	$0, R2
 storeloop:
 	LDREX	(R0), R4          // loads R4
-	STREX	R1, (R0), R1      // stores R2
+	STREX	R2, (R0), R1      // stores R2
 	CMP	$0, R1
 	BNE	storeloop
 	JMP	exit1<>(SB)

src/runtime/sys_freebsd_arm.s

@@ -86,10 +86,10 @@ TEXT runtime·exit(SB),NOSPLIT,$-8
 TEXT runtime·exitThread(SB),NOSPLIT,$0-4
 	MOVW	wait+0(FP), R0
 	// We're done using the stack.
-	MOVW	$0, R1
+	MOVW	$0, R2
 storeloop:
 	LDREX	(R0), R4          // loads R4
-	STREX	R1, (R0), R1      // stores R2
+	STREX	R2, (R0), R1      // stores R2
 	CMP	$0, R1
 	BNE	storeloop
 	MOVW	$0, R0		// arg 1 long *state

src/runtime/sys_netbsd_arm.s

@@ -22,10 +22,10 @@ TEXT runtime·exit(SB),NOSPLIT,$-4
 TEXT runtime·exitThread(SB),NOSPLIT,$0-4
 	MOVW wait+0(FP), R0
 	// We're done using the stack.
-	MOVW $0, R1
+	MOVW $0, R2
 storeloop:
 	LDREX (R0), R4          // loads R4
-	STREX R1, (R0), R1      // stores R2
+	STREX R2, (R0), R1      // stores R2
 	CMP $0, R1
 	BNE storeloop
 	SWI $0xa00136	// sys__lwp_exit

src/runtime/sys_openbsd_arm.s

@@ -26,10 +26,10 @@ TEXT runtime·exit(SB),NOSPLIT,$-4
 TEXT runtime·exitThread(SB),NOSPLIT,$0-4
 	MOVW	wait+0(FP), R0
 	// We're done using the stack.
-	MOVW	$0, R1
+	MOVW	$0, R2
 storeloop:
 	LDREX	(R0), R4          // loads R4
-	STREX	R1, (R0), R1      // stores R2
+	STREX	R2, (R0), R1      // stores R2
 	CMP	$0, R1
 	BNE	storeloop
 	MOVW	$0, R0			// arg 1 - notdead