Add support for MIPS .rld_map section.

Add support for MIPS .rld_map section.

Includes DT_MIPS_RLD_MAP and DT_MIPS_RLD_MAP_REL dynamic tags and
__RLD_MAP symbol.

2016-06-20  Vladimir Radosavljevic  <Vladimir.Radosavljevic@imgtec.com>

elfcpp/
        * elfcpp.h (DT_MIPS_RLD_MAP_REL): New enum constant.
gold/
        * mips.cc (Target_mips::Target_mips): Initialize rld_map_.
        (Target_mips::rld_map_): New data member.
        (Target_mips::do_finalize_sections): Add support for
        DT_MIPS_RLD_MAP and DT_MIPS_RLD_MAP_REL dynamic tags,
        .rld_map section, and __RLD_MAP symbol.
        (Target_mips::do_dynamic_tag_custom_value): Add support for
        DT_MIPS_RLD_MAP_REL dynamic tag.
        * output.cc (Output_data_dynamic::get_entry_offset): New method
        definition.
        * output.h (Output_data_dynamic::get_entry_offset): New method
        declaration.