This commit adds support to read AIX big archive inside internal/xcoff
package.

Change-Id: I4317b40824b24312a69c918dfc6438dc3aff7be7
Reviewed-on: https://go-review.googlesource.com/c/153398
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>