os: set TMPDIR on darwin/arm

This is a roll forward of 2adc3bd6ef. It occurred to me that we will
want this code on both darwin/arm and darwin/arm64. Removing _arm from
the file name conveniently avoids #10032.

Change-Id: I3a96a3e7020907d9307af8f696e26ad55b2060f0
Reviewed-on: https://go-review.googlesource.com/6460
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: David Crawshaw <crawshaw@golang.org>

src/os/file_darwin.go

@@ -0,0 +1,40 @@
+// Copyright 2015 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// +build arm arm64
+
+package os
+
+/*
+#cgo CFLAGS: -x objective-c
+#cgo LDFLAGS: -framework CoreFoundation -framework Foundation
+
+#include <sys/param.h>
+#include <CoreFoundation/CFString.h>
+#include <Foundation/NSPathUtilities.h>
+
+char tmpdir[MAXPATHLEN];
+
+char* loadtmpdir() {
+	tmpdir[0] = 0;
+	CFStringRef path = (CFStringRef)NSTemporaryDirectory();
+	CFStringGetCString(path, tmpdir, sizeof(tmpdir), kCFStringEncodingUTF8);
+	return tmpdir;
+}
+*/
+import "C"
+
+func init() {
+	if Getenv("TMPDIR") != "" {
+		return
+	}
+	dir := C.GoString(C.loadtmpdir())
+	if len(dir) == 0 {
+		return
+	}
+	if dir[len(dir)-1] == '/' {
+		dir = dir[:len(dir)-1]
+	}
+	Setenv("TMPDIR", dir)
+}