runtime/cgo: move TMPDIR magic out of os

It's not clear this really belongs anywhere at all,
but this is a better place for it than package os.
This way package os can avoid importing "C".

Fixes #10455.

Change-Id: Ibe321a93bf26f478951c3a067d75e22f3d967eb7
Reviewed-on: https://go-review.googlesource.com/12574Reviewed-by: David Crawshaw <crawshaw@golang.org>
Reviewed-by: Dave Cheney <dave@cheney.net>

src/os/file_darwin.go → src/runtime/cgo/tmpdir_darwin.go

@@ -4,7 +4,7 @@
 
 // +build arm arm64
 
-package os
+package cgo
 
 /*
 #cgo CFLAGS: -x objective-c