Enhance gdb.lookup_objfile so that it works with a symlinked binary.

Enhance gdb.lookup_objfile so that it works with a symlinked binary.

gdb/Changelog:

	* objfiles.c (objfile_filename): New function.
	* objfiles.h (objfile_filename): Declare it.
	(objfile_name): Add function comment.
	* python/py-objfile.c (objfpy_lookup_objfile_by_name): Try both the
	bfd file name (which may be realpath'd), and the original name.

gdb/testsuite/ChangeLog:

	* gdb.python/py-objfile.exp: Test gdb.lookup_objfile on symlinked
	binary.