New python function gdb.lookup_objfile.

New python function gdb.lookup_objfile.

gdb/ChangeLog:

	* NEWS: Mention gdb.lookup_objfile.
	* python/python.c (GdbMethods): Add lookup_objfile.
	* python/python-internal.h (gdbpy_lookup_objfile): Declare.
	* python/py-objfile.c: #include "symtab.h".
	(objfpy_build_id_ok, objfpy_build_id_matches): New functions.
	(objfpy_lookup_objfile_by_name): New function.
	(objfpy_lookup_objfile_by_build_id): New function.
	(gdbpy_lookup_objfile): New function.

gdb/doc/ChangeLog:

	* python.texi (Objfiles In Python): Document gdb.lookup_objfile.

gdb/testsuite/ChangeLog:

	* lib/gdb-python.exp (get_python_valueof): New function.
	* gdb.python/py-objfile.exp: Add tests for gdb.lookup_objfile.