Refactor Sized_relobj_file::do_relocate_sections.

gold/
	* aarch64.cc (AArch64_relobj::do_relocate_sections): Call
	Sized_relobj_file::relocate_section_range().
	* arm.cc (Arm_relobj::do_relocate_sections): Likewise.
	* object.h (Sized_relobj_file::relocate_section_range): New method.
	* reloc.cc (Sized_relobj_file::do_relocate_sections): Move
	implementation...
	(Sized_relobj_file::relocate_section_range): ...to new method.