This commit adds the new package cmd/link/internal/loadxcoff.

It also adds a new symbol kind in cmd/link/internal/sym package, which
aims to represent TOC entries for XCOFF files.

cmd/dist is updated to add this new package and cmd/internal/xcoff during
the bootstrap.

Updates: #25893

Change-Id: I42b6578cf0ba4cc28ad4aa98122a91ab1d1bbf6e
Reviewed-on: https://go-review.googlesource.com/c/138728
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>