Add the ability to the BFD library to read build-ids from core flies.

Add the ability to the BFD library to read build-ids from core flies.

	* elf-bfd.h (elf_backend_data) <elf_backend_core_find_build_id>:
	New field.
	(_bfd_elf32_core_find_build_id, _bfd_elf64_core_find_build_id):
	New functions.
	(elf_read_notes): Add declaration.
	* elf.c (elf_read_notes): Move elf-bfd.h.
	(_bfd_elf_core_find_build_id): New function.
	(bfd_section_from_phdr): Scan core file PT_LOAD segments for
	build-id if none is known.
	(elf_parse_notes): For core files, scan for notes.
	* elfcore.h (elf_core_file_matches_executable_p): If both
	BFDs have identical build-ids, then they match.
	(_bfd_elf_core_find_build_id): New function.
	* elfxx-target.h (elf_backend_core_find_build_id): Define.
	(elfNN_bed): Add elf_backend_core_find_build_id.