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.
Reviewed-on: https://go-review.googlesource.com/72250
From-SVN: r253955
001cbba0ef