Add support for R_MIPS_HIGHER/HIGHEST, R_MICROMIPS_HIGHER/HIGHEST relocations.

Add support for R_MIPS_HIGHER/HIGHEST, R_MICROMIPS_HIGHER/HIGHEST relocations.

2017-02-15  Vladimir Radosavljevic  <Vladimir.Radosavljevic@imgtec.com>

        PR gold/21111
        * mips.cc (Mips_relocate_functions::relhigher): New method.
        (Mips_relocate_functions::relhighest): Likewise.
        (mips_get_size_for_reloc): Add support for relocs: R_MIPS_HIGHER and
        R_MIPS_HIGHEST.
        (Target_mips::Scan::local): Add support for relocs: R_MIPS_HIGHER,
        R_MIPS_HIGHEST, R_MICROMIPS_HIGHER and R_MICROMIPS_HIGHEST.
        (Target_mips::Scan::global): Likewise.
        (Target_mips::Scan::get_reference_flags): Likewise.
        (Target_mips::Relocate::relocate): Call static methods for resolving
        HIGHER and HIGHEST relocations.