runtime: small Native Client fixes

mem_nacl.c: add SysFault
env_posix.c: add nacl to build tag, from CL 15790043

LGTM=remyoudompheng, iant
R=rsc, remyoudompheng, iant
CC=golang-codereviews
https://golang.org/cl/72780043

src/pkg/runtime/env_posix.c

@@ -2,7 +2,7 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-// +build darwin dragonfly freebsd linux netbsd openbsd solaris windows
+// +build darwin dragonfly freebsd linux nacl netbsd openbsd solaris windows
 
 #include "runtime.h"
 #include "arch_GOARCH.h"

src/pkg/runtime/mem_nacl.c

@@ -53,6 +53,12 @@ runtime·SysFree(void *v, uintptr n, uint64 *stat)
 	runtime·munmap(v, n);
 }
 
+void
+runtime·SysFault(void *v, uintptr n)
+{
+	runtime·mmap(v, n, PROT_NONE, 0, -1, 0);
+}
+
 void*
 runtime·SysReserve(void *v, uintptr n)
 {