cmd/go: support cgo files in overlays

This requires rewriting the paths of the files passed to the cgo tool
toolchain to use the overlaid paths instead of the disk paths of
files. Because the directories of the overlaid paths don't exist in
general, the cgo tool have been updated to run in base.Cwd instead of
the package directory.

For #39958

Change-Id: I8986de889f56ecc2e64fa69f5f6f29fa907408f9
Reviewed-on: https://go-review.googlesource.com/c/go/+/262618


Trust: Michael Matloob <matloob@golang.org>
Run-TryBot: Michael Matloob <matloob@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Jay Conrod <jayconrod@google.com>
Reviewed-by: Bryan C. Mills <bcmills@google.com>