This commit adds support to read XCOFF files and AIX big archives in
go/internal/gccgoimporter.

Fixes: #29113

Change-Id: Id84d40358ff98fae5a576d1ebdd65980896365b9
Reviewed-on: https://go-review.googlesource.com/c/152720
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>