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.

For #37272

Change-Id: I64a1e7ca1c84348efcbf1cf62049eeb05c830ed8
Reviewed-on: https://go-review.googlesource.com/c/go/+/259298


Trust: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Than McIntosh <thanm@google.com>