cmd/go, cmd/cgo: update gofrontend mangling checks

This is a port of two patches in the master repository.

https://golang.org/cl/259298

    cmd/cgo: split gofrontend mangling checks into cmd/internal/pkgpath

    This is a step toward porting https://golang.org/cl/219817 from the
    gofrontend repo to the main repo.

    Note that this also corrects the implementation of the v2 mangling
    scheme to use ..u and ..U where appropriate.

https://golang.org/cl/259299

    cmd/go: use cmd/internal/pkgpath for gccgo pkgpath symbol

For golang/go#37272
For golang/go#41862

Change-Id: Id276ca1bac7e1d850408a9d222c19e57b26ba001
Reviewed-on: https://go-review.googlesource.com/c/gofrontend/+/270637
Trust: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Than McIntosh <thanm@google.com>