elf: Add _bfd_elf_link_m[un]map_section_contents

To copy input section contents, add _bfd_elf_link_mmap_section_contents
and _bfd_elf_link_munmap_section_contents to mmap in the input sections.

	* elf-bfd.h (_bfd_elf_link_mmap_section_contents): New.
	(_bfd_elf_link_munmap_section_contents): Likewise.
	* elf.c (elf_mmap_section_contents): New.
	(_bfd_elf_mmap_section_contents): Use it.
	(_bfd_elf_link_mmap_section_contents): New.
	(_bfd_elf_link_munmap_section_contents): Likewise.
	* elflink.c (elf_link_input_bfd): Call
	_bfd_elf_link_mmap_section_contents instead of
	bfd_get_full_section_contents.  Call
	_bfd_elf_link_munmap_section_contents to munmap the section
	contents.
	(bfd_elf_final_link): When mmap is used, initialize
	max_contents_size to _bfd_minimum_mmap_size and increase it
	for compressed or linker created sections or sections whose
	rawsize != size.