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