runtime: darwin/amd64, don't depend on outarg slots being unmodified

sigtramp was calling sigtrampgo and depending on the fact that
the 3rd argument slot will not be modified on return.  Our calling
convention doesn't guarantee that.  Avoid that assumption.

There's no actual bug here, as sigtrampgo does not in fact modify its
argument slots.  But I found this while working on the dead stack slot
clobbering tool.  https://go-review.googlesource.com/c/23924/

Change-Id: Ia7e791a2b4c1c74fff24cba8169e7840b4b06ffc
Reviewed-on: https://go-review.googlesource.com/36216
Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/runtime/sys_darwin_amd64.s

@@ -252,14 +252,15 @@ TEXT runtime·sigfwd(SB),NOSPLIT,$0-32
 	POPQ	BP
 	RET
 
-TEXT runtime·sigtramp(SB),NOSPLIT,$32
+TEXT runtime·sigtramp(SB),NOSPLIT,$40
 	MOVL SI, 24(SP) // save infostyle for sigreturn below
+	MOVQ R8, 32(SP) // save ctx
 	MOVL DX, 0(SP)  // sig
 	MOVQ CX, 8(SP)  // info
 	MOVQ R8, 16(SP) // ctx
 	MOVQ $runtime·sigtrampgo(SB), AX
 	CALL AX
-	MOVQ 16(SP), DI // ctx
+	MOVQ 32(SP), DI // ctx
 	MOVL 24(SP), SI // infostyle
 	MOVL $(0x2000000+184), AX
 	SYSCALL