runtime: align stack in sigfwd for darwin/386

We might be forwarding to a C signal handler.  C code expects the stack
to be aligned.  Should fix darwin/386 build: the testcarchive tests were
hanging as the program got an endless series of SIGSEGV signals.

Change-Id: Ia02485d3736a3c40e12259f02d25f842cf8e4d29
Reviewed-on: https://go-review.googlesource.com/19025
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: David Crawshaw <crawshaw@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/runtime/sys_darwin_386.s

@@ -242,15 +242,21 @@ TEXT runtime·sigaction(SB),NOSPLIT,$0
 	MOVL	$0xf1, 0xf1  // crash
 	RET
 
-TEXT runtime·sigfwd(SB),NOSPLIT,$12-16
-	MOVL	sig+4(FP), AX
-	MOVL	AX, 0(SP)
-	MOVL	info+8(FP), AX
-	MOVL	AX, 4(SP)
-	MOVL	ctx+12(FP), AX
-	MOVL	AX, 8(SP)
+TEXT runtime·sigfwd(SB),NOSPLIT,$0-16
 	MOVL	fn+0(FP), AX
+	MOVL	sig+4(FP), BX
+	MOVL	info+8(FP), CX
+	MOVL	ctx+12(FP), DX
+	MOVL	SP, SI
+	SUBL	$32, SP		// align stack; handler might be C code
+	ANDL	$~15, SP
+	MOVL	BX, 0(SP)
+	MOVL	CX, 4(SP)
+	MOVL	DX, 8(SP)
+	MOVL	SI, 12(SP)
 	CALL	AX
+	MOVL	12(SP), AX
+	MOVL	AX, SP
 	RET
 
 TEXT runtime·sigreturn(SB),NOSPLIT,$12-8