runtime: correct exitThread on openbsd/arm64

The notdead argument to sys___threxit() is a pointer, hence requires a 64-bit
move rather than a 32-bit one.

Updates #31656

Change-Id: I52ad31ed5afaf43ccc3d934025288216e8052528
Reviewed-on: https://go-review.googlesource.com/c/go/+/174124Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/runtime/sys_openbsd_arm64.s

@@ -25,7 +25,7 @@ TEXT runtime·exit(SB),NOSPLIT|NOFRAME,$0
 
 // func exitThread(wait *uint32)
 TEXT runtime·exitThread(SB),NOSPLIT,$0-4
-	MOVW	wait+0(FP), R0		// arg 1 - notdead
+	MOVD	wait+0(FP), R0		// arg 1 - notdead
 	MOVD	$302, R8		// sys___threxit
 	SVC
 	MOVD	$0, R0			// crash on syscall failure