Revert "os: set TMPDIR on darwin/arm"

This reverts commit 87a0d395.

Looks like introducing file_darwin_arm.go is confusing something in the API checker (probably go/types) into ignoring file.go, so the O_SYNC symbol is being lost.

No actual bug in this CL AFAIK, but I'll fix the other bug later and then roll this forward.

Change-Id: Ic132fb101e4b5f2716f7a0d15872bf35bdf42139
Reviewed-on: https://go-review.googlesource.com/6331Reviewed-by: David Crawshaw <crawshaw@golang.org>

src/os/file_darwin_arm.go

-// 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.
-
-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("TEMPDIR") != "" {
-		return
-	}
-	dir := C.GoString(C.loadtmpdir())
-	if len(dir) == 0 {
-		return
-	}
-	if dir[len(dir)-1] == '/' {
-		dir = dir[:len(dir)-1]
-	}
-	Setenv("TMPDIR", dir)
-}