[gold][aarch64]Give informative error message when stub-group-size is too large

[Cary Coutant]

> 2014-12-10  Jing Yu  <jingyu@google.com>
>
>       * aarch64.cc (Target_aarch64): Add new variable: stub_group_size_.
>       (AArch64_relocate_functions::maybe_apply_stub): Add new parameter.
>       Update error message.
>       (Target_aarch64::do_relax): Use absolute value of option
> stub_group_size. Replace local variable with class member
> stub_group_size_.

This is OK. Thanks!

-cary