runtime: fix SP adjustment on amd64p32

On amd64p32, rt0_go attempts to reserve 128 bytes of scratch space on
the stack, but due to a register mixup this ends up being a no-op. Fix
this so we actually reserve the stack space.

Change-Id: I04dbfbeb44f3109528c8ec74e1136bc00d7e1faa
Reviewed-on: https://go-review.googlesource.com/32331
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/runtime/asm_amd64p32.s

@@ -12,7 +12,7 @@ TEXT runtime·rt0_go(SB),NOSPLIT,$0
 	MOVL	argc+0(FP), AX
 	MOVL	argv+4(FP), BX
 	MOVL	SP, CX
-	SUBL	$128, SP		// plenty of scratch
+	SUBL	$128, CX		// plenty of scratch
 	ANDL	$~15, CX
 	MOVL	CX, SP