[gold patch] Add method to get output section address that works with --just-symbols

[Cary Coutant]

I've committed this patch to fix a problem where a branch directly to
a symbol defined in a module linked with --just-symbols can result in
a segfault.

Tested on ppc64 and x86_64.

-cary


2013-07-31  Cary Coutant  <ccoutant@google.com>

        * object.cc (Sized_relobj::do_output_section_address): New function.
        (Sized_relobj): Instantiate explicitly.
        * object.h (Object::output_section_address): New function.
        (Object::do_output_section_address): New function.
        (Sized_relobj::do_output_section_address): New function.
        * powerpc.cc (Target_powerpc::symval_for_branch): Use it.