This commit adds a new package in cmd/internal which aims
to debug and load XCOFF files.

Updates: #25893, #28037

Change-Id: I47db495bedfa43e9129a831b9b8bbc35b703567b
Reviewed-on: https://go-review.googlesource.com/c/138727
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>