runtime: fix/improve exitThread on openbsd

OpenBSD's __threxit syscall takes a pointer to a 32-bit value that will be
zeroed immediately before the thread exits. Make use of this instead of
zeroing freeWait from the exitThread assembly and using hacks like switching
to a static stack, so this works on 386.

Change-Id: I3ec5ead82b6496404834d148f713794d5d9da723
Reviewed-on: https://go-review.googlesource.com/105055Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Austin Clements <austin@google.com>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/runtime/sys_openbsd_386.s

@@ -19,17 +19,8 @@ TEXT runtime·exit(SB),NOSPLIT,$-4
 	MOVL	$0xf1, 0xf1		// crash
 	RET
 
-GLOBL exitStack<>(SB),RODATA,$8
-DATA exitStack<>+0x00(SB)/4, $0
-DATA exitStack<>+0x04(SB)/4, $0
-
 // func exitThread(wait *uint32)
 TEXT runtime·exitThread(SB),NOSPLIT,$0-4
-	MOVL	wait+0(FP), AX
-	// We're done using the stack.
-	MOVL	$0, (AX)
-	// sys__lwp_exit takes 1 argument, which it expects on the stack.
-	MOVL	$exitStack<>(SB), SP
 	MOVL	$302, AX		// sys___threxit
 	INT	$0x80
 	MOVL	$0xf1, 0xf1		// crash

src/runtime/sys_openbsd_amd64.s

@@ -90,10 +90,7 @@ TEXT runtime·exit(SB),NOSPLIT,$-8
 
 // func exitThread(wait *uint32)
 TEXT runtime·exitThread(SB),NOSPLIT,$0-8
-	MOVQ	wait+0(FP), AX
-	// We're done using the stack.
-	MOVL	$0, (AX)
-	MOVQ	$0, DI			// arg 1 - notdead
+	MOVQ	wait+0(FP), DI		// arg 1 - notdead
 	MOVL	$302, AX		// sys___threxit
 	SYSCALL
 	MOVL	$0xf1, 0xf1		// crash

src/runtime/sys_openbsd_arm.s

@@ -24,15 +24,7 @@ TEXT runtime·exit(SB),NOSPLIT|NOFRAME,$0
 
 // func exitThread(wait *uint32)
 TEXT runtime·exitThread(SB),NOSPLIT,$0-4
-	MOVW	wait+0(FP), R0
-	// We're done using the stack.
-	MOVW	$0, R2
-storeloop:
-	LDREX	(R0), R4          // loads R4
-	STREX	R2, (R0), R1      // stores R2
-	CMP	$0, R1
-	BNE	storeloop
-	MOVW	$0, R0			// arg 1 - notdead
+	MOVW	wait+0(FP), R0		// arg 1 - notdead
 	MOVW	$302, R12		// sys___threxit
 	SWI	$0
 	MOVW.CS	$1, R8			// crash on syscall failure