misc/cgo/test: fix test on darwin/386 with cgo enabled

Apparently the darwin/386 builder does not enable cgo.
This failure turned up running

	GOARCH=386 GOHOSTARCH=386 ./all.bash

on my Mac.

Change-Id: Ia2487c4fd85d4b0f9f564880f22d9fde379946c3
Reviewed-on: https://go-review.googlesource.com/18859Reviewed-by: Ian Lance Taylor <iant@golang.org>

misc/cgo/test/sigaltstack.go

@@ -30,7 +30,7 @@ static void changeSignalStack() {
 }
 
 static void restoreSignalStack() {
-#if defined(__x86_64__) && defined(__APPLE__)
+#if (defined(__x86_64__) || defined(__i386__)) && defined(__APPLE__)
 	// The Darwin C library enforces a minimum that the kernel does not.
 	// This is OK since we allocated this much space in mpreinit,
 	// it was just removed from the buffer by stackalloc.