cmd/cgo, cmd/internal/pkgpath: support gofrontend mangler v3
The gofrontend mangling scheme used by gccgo and GoLLVM has changed again.
Support the new version. This is a port of the relevant parts of
https://golang.org/cl/271726.
For #41862
Change-Id: I9c961c8e17ec960a83a23e1d49ea900962b63393
Reviewed-on: https://go-review.googlesource.com/c/go/+/272127
Trust: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org> Reviewed-by: Than McIntosh <thanm@google.com>
TryBot-Result: Go Bot <gobot@golang.org>