runtime, syscall: address vet errors in openbsd/arm64 assembly

Updates #31656

Change-Id: Ie28734298bf1a2d5243f1ac15569311c1887176e
Reviewed-on: https://go-review.googlesource.com/c/go/+/174126Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/runtime/sys_openbsd_arm64.s

@@ -24,7 +24,7 @@ TEXT runtime·exit(SB),NOSPLIT|NOFRAME,$0
 	RET
 
 // func exitThread(wait *uint32)
-TEXT runtime·exitThread(SB),NOSPLIT,$0-4
+TEXT runtime·exitThread(SB),NOSPLIT,$0
 	MOVD	wait+0(FP), R0		// arg 1 - notdead
 	MOVD	$302, R8		// sys___threxit
 	SVC

src/syscall/asm_openbsd_arm64.s

@@ -65,7 +65,7 @@ TEXT ·Syscall9(SB),NOSPLIT,$0-104
 	MOVD	a8+64(FP), R7
 	MOVD	a9+72(FP), R8	// on stack
 	MOVD	R8, 8(RSP)
-	MOVD	trap+0(FP), R8	// syscall number
+	MOVD	num+0(FP), R8	// syscall number
 	SVC
 	BCC	ok
 	MOVD	$-1, R4