[gold][powerpc] PR gold/20529 - relaxing loop never ends

[Cary Coutant]

>     2016-08-26  Han Shen  <shenhan@google.com>
>
>         PR gold/20529 - relaxing loop never ends.
>
>         * powerpc.cc (Stub_table::min_size_threshold_): New member to
>         limit size.
>         (Stub_table::set_min_size_threshold): New member function.
>         (Stub_table::set_address_and_size): Add code to only allow size
>         increase.
>         (Target_powerpc::do_relax): Add code to record last size.

This is OK. Thanks!

Do you think it's serious enough to warrant a backport to 2.27?

-cary