elf: Add _bfd_elf_m[un]map_section_contents

elf: Add _bfd_elf_m[un]map_section_contents

Add _bfd_elf_mmap_section_contents and _bfd_elf_munmap_section_contents.
A backend must opt-in to use mmap.  It should replace

bfd_malloc_and_get_section -> _bfd_elf_mmap_section_contents
free			   -> _bfd_elf_munmap_section_contents

on section contents.

	* compress.c (bfd_get_full_section_contents): Don't allocate
	buffer if mmapped_p is true.
	* elf-bfd.h (elf_backend_data): Add use_mmap.
	(bfd_elf_section_data): Add contents_addr and contents_size.
	(_bfd_elf_mmap_section_contents): New.
	(_bfd_elf_munmap_section_contents): Likewise.
	* elf-eh-frame.c (_bfd_elf_parse_eh_frame): Replace
	bfd_malloc_and_get_section and free with
	_bfd_elf_mmap_section_contents and _bfd_elf_munmap_section_contents
	on section contents.
	* elf-sframe.c (_bfd_elf_parse_sframe): Likewise.
	* elf.c (_bfd_elf_make_section_from_shdr): Replace
	bfd_malloc_and_get_section and free with
	_bfd_elf_mmap_section_contents and _bfd_elf_munmap_section_contents
	on section contents.
	(_bfd_elf_print_private_bfd_data): Likewise.
	(_bfd_elf_mmap_section_contents): New.
	(_bfd_elf_munmap_section_contents): Likewise.
	* elf32-i386.c (elf_i386_scan_relocs): Replace
	bfd_malloc_and_get_section and free with
	_bfd_elf_mmap_section_contents and _bfd_elf_munmap_section_contents
	on section contents.
	* elf64-x86-64.c (elf_x86_64_scan_relocs): Likewise.
	(elf_x86_64_get_synthetic_symtab): Likewise.
	* elfcode.h (elf_checksum_contents): Likewise.
	* elflink.c (elf_link_add_object_symbols): Likewise.
	(bfd_elf_get_bfd_needed_list): Likewise.
	* elfxx-target.h (elf_backend_use_mmap): New.
	(elfNN_bed): Add elf_backend_use_mmap.
	* elfxx-x86.c (elf_x86_size_or_finish_relative_reloc): Replace
	bfd_malloc_and_get_section and free with
	_bfd_elf_mmap_section_contents and _bfd_elf_munmap_section_contents
	on section contents.
	(_bfd_x86_elf_get_synthetic_symtab): Replace free with
	_bfd_elf_munmap_section_contents.
	* elfxx-x86.h (elf_backend_use_mmap): New.
	* libbfd.c: Include "elf-bfd.h".
	(_bfd_generic_get_section_contents): Call bfd_mmap_local for
	mmapped_p.
	* opncls.c (_bfd_delete_bfd): Also munmap ELF section contents.
	* section.c (asection): Add mmapped_p.
	(BFD_FAKE_SECTION): Updated.
	(bfd_malloc_and_get_section): Add a sanity check for not
	mmapped_p.
	* bfd-in2.h: Regenerated.