Refactor Output_data_reloc_base::do_write for MIPS-specific relocs.

Refactor Output_data_reloc_base::do_write for MIPS-specific relocs.

This patch is a simple refactoring that will allow the MIPS backend to
replace the Output_data_reloc_base::do_write() method without copying
its entire implementation. I've moved the implementation of do_write()
into a function template, which can be instantiated with a custom
class to write the MIPS-specific relocation format. The custom class
for MIPS needs access to the symbol index and address from
Output_reloc, so I've included the part of Vlad's MIPS-64 patch that
makes those accessor methods public.

2016-03-08  Cary Coutant  <ccoutant@gmail.com>
            Vladimir Radosavljevic  <vladimir.radosavljevic@imgtec.com>

gold/
	* output.cc (Output_reloc_writer): New type.
	(Output_data_reloc_base::do_write): Move implementation to template
	in output.h and replace with invocation of template.
	* output.h (Output_file): Move to top of file.
	(Output_reloc::get_symbol_index): Move to public interface.
	(Output_reloc::get_address): Likewise.
	(Output_data_reloc_base::do_write_generic): New function template.