Add support for locating separate debug info files via the build-id method.

Add support for locating separate debug info files via the build-id method.

	PR binutils/20876
bfd	* opncls.c (find_separate_debug_file): Add include_dirs
	parameter.  Only include the directory part of the bfd's filename
	in search paths if include_dirs is true.  Add a couple of extra
	locations for looking for debug files.
	( bfd_follow_gnu_debuglink): Update invocation of
	find_separate_debug_file.
	(bfd_follow_gnu_debugaltlink): Likewise.
	(get_build_id): New function: Finds the build-id of the given bfd.
	(get_build_id_name): New function: Computes the name of the
	separate debug info file for a bfd, based upon its build-id.
	(check_build_id_file): New function: Checks to see if a separate
	debug info file exists at the given location, and that its
	build-id matches that of the original bfd.
	(bfd_follow_build_id_debuglink): New function: Finds a separate
	debug info file for a given bfd by using the build-id method.
	* dwarf2.c (_bfd_dwarf2_slurp_debug_info): Try using the build-id
	method of locating a separate debug info file before using the
	debuglink method.
	* bfd-in2.h: Regenerate.

binutils * NEWS: Mention the new feature.
	* testsuite/binutils-all/objdump.exp (test_build_id_debuglink):
	New proc to test the location of separate debug info files using
	the build-id method.