runtime: silence darwin/386 build warnings

R=golang-dev, bradfitz
CC=golang-dev
https://golang.org/cl/5502056

src/pkg/runtime/cgo/gcc_darwin_386.c

@@ -2,6 +2,7 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
+#include <string.h> /* for strerror */
 #include <pthread.h>
 #include "libcgo.h"
 

src/pkg/runtime/signal_darwin_386.c

@@ -142,7 +142,7 @@ sigaction(int32 i, void (*fn)(int32, Siginfo*, void*, G*), bool restart)
 		sa.sa_flags |= SA_RESTART;
 	sa.sa_mask = ~0U;
 	sa.sa_tramp = (void*)runtime·sigtramp;	// runtime·sigtramp's job is to call into real handler
-	*(uintptr*)&sa.__sigaction_u = (uintptr)fn;
+	*(uintptr*)sa.__sigaction_u = (uintptr)fn;
 	runtime·sigaction(i, &sa, nil);
 }