runtime: use correct register in darwin/386 pipe_trampoline

Updates #31264

Change-Id: I745744dd3fdaa432d70e8dc9336547017bac89ee
Reviewed-on: https://go-review.googlesource.com/c/go/+/184377
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Elias Naur <mail@eliasnaur.com>

src/runtime/sys_darwin_386.s

@@ -88,7 +88,7 @@ TEXT runtime·pipe_trampoline(SB),NOSPLIT,$0
 	PUSHL	BP
 	MOVL	SP, BP
 	SUBL	$8, SP
-	MOVL	16(SP), CX		// arg 1 pipefd
+	MOVL	16(SP), AX		// arg 1 pipefd
 	MOVL	AX, 0(SP)
 	CALL	libc_pipe(SB)
 	TESTL	AX, AX