New python attribute gdb.Objfile.build_id.

New python attribute gdb.Objfile.build_id.

gdb/ChangeLog:

	* NEWS: Mention gdb.Objfile.build_id.
	* build-id.c (build_id_bfd_get): Make non-static.
	* build-id.h (build_id_bfd_get): Add declaration.
	* python/py-objfile.c: #include "build-id.h", "elf-bfd.h".
	(OBJFPY_REQUIRE_VALID): New macro.
	(objfpy_get_build_id): New function.
	(objfile_getset): Add "build_id".
	* utils.c (make_hex_string): New function.
	* utils.h (make_hex_string): Add declaration.

gdb/doc/ChangeLog:

	* python.texi (Objfiles In Python): Document Objfile.build_id.

gdb/testsuite/ChangeLog:

	* lib/gdb.exp (get_build_id): New function.
	(build_id_debug_filename_get): Rewrite to use it.
	* gdb.python/py-objfile.exp: Add test for objfile.build_id.