This commit moves cmd/internal/xcoff package to internal/xcoff because
it will be needed to add XCOFF support in go/internal/gccgoimporter.

Change-Id: Id12df0c438fb7db4a6a458fc1478480851bf7771
Reviewed-on: https://go-review.googlesource.com/c/152719
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>