Correct the definition of _gp and _GLOBAL_OFFSET_TABLE_ symbols for MIPS.

Correct the definition of _gp and _GLOBAL_OFFSET_TABLE_ symbols for MIPS.

gold/
        * mips.cc (symbol_refs_local): Return false if a symbol
        is from a dynamic object.
        (Target_mips::got_section): Make _GLOBAL_OFFSET_TABLE_ STV_HIDDEN.
        (Target_mips::set_gp): Refactor.  Make _gp STT_NOTYPE and
        STB_LOCAL.
        (Target_mips::do_finalize_sections): Set _gp after all the checks
        for creating .got are done.
        (Target_mips::Scan::global): Remove unused code.