cmd/go/internal/cache: implement build artifact cache

cmd/go/internal/cache: implement build artifact cache

The cache is stored in $GOCACHE, which is printed by go env and
defaults to a subdirectory named "go-build" in the standard user cache
directory for the host operating system.

This CL only implements the cache. Future CLs will store data in it.

Change-Id: I0b4965a9e50f852e17e44ec3d6dafe05b58f0d22
Reviewed-on: https://go-review.googlesource.com/68116
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: David Crawshaw <crawshaw@golang.org>