Add NaCl support to gold for Mips

[Cary Coutant]

> I added ChangeLog entry.

Thanks. I've committed the patch on your behalf.

2014-09-03  Sasa Stankovic  <Sasa.Stankovic@imgtec.com>

        * mips.cc (Target_mips_nacl): New class.
        (Target_selector_mips_nacl): New class.
        (target_selector_mips32): Rename from target_selector_mips32be and use
        Target_selector_mips_nacl instead of Target_selector_mips.
        (target_selector_mips32el): Rename from target_selector_mips32 and use
        Target_selector_mips_nacl instead of Target_selector_mips.
        (target_selector_mips64): Rename from target_selector_mips64be and use
        Target_selector_mips_nacl instead of Target_selector_mips.
        (target_selector_mips64el): Rename from target_selector_mips64 and use
        Target_selector_mips_nacl instead of Target_selector_mips.
        (Target_mips::mips_info): Add const attribute.

-cary