Adds a function that returns an OS-dependent location
for user-specific cache data.

Fixes golang/go#22536

Change-Id: Ifff015452494571ad357fa2d945d66a5992c751d
Reviewed-on: https://go-review.googlesource.com/78835
Run-TryBot: Andrew Bonventre <andybons@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>