debug/dwarf: support 64-bit DWARF in byte order check

Also fix 64-bit DWARF to read a 64-bit abbrev offset in the
compilation unit.

This is a backport of https://golang.org/cl/71171, which will be in
the Go 1.10 release, to the gofrontend copy. Doing it now because AIX
is pretty much the only system that uses 64-bit DWARF.

Change-Id: I707bc127e8413c2ee8d63abf8c73ee4f3bd98f8a
Reviewed-on: https://go-review.googlesource.com/72250
Reviewed-by: Than McIntosh <thanm@google.com>
5 files changed