This commit adds support for DWARF 64bits which is needed for AIX
operating system.

It also adds the save of each compilation unit's size which will be
used during XCOFF generation in a following patch.

Updates: #25893

Change-Id: Icdd0a4dd02bc0a9f0df319c351fb1db944610015
Reviewed-on: https://go-review.googlesource.com/c/138729
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>