Add Objfile.lookup_{global,static}_symbol functions

Add Objfile.lookup_{global,static}_symbol functions

This is essentially the inverse of Symbol.objfile. This allows
handling different symbols with the same name (but from different
objfiles) and can also be faster if the objfile is known.

gdb/ChangeLog:

2019-07-29  Christian Biesinger  <cbiesinger@google.com>

	* NEWS: Mention new functions Objfile.lookup_{global,static}_symbol.
	* python/py-objfile.c (objfpy_lookup_global_symbol): New function.
	(objfpy_lookup_static_symbol): New function.
	(objfile_object_methods): Add new functions.

gdb/doc/ChangeLog:

2019-07-29  Christian Biesinger  <cbiesinger@google.com>

	* python.texi (Objfiles In Python): Document new functions
	  Objfile.lookup_{global,static}_symbol.

gdb/testsuite/ChangeLog:

2019-07-29  Christian Biesinger  <cbiesinger@google.com>

	* gdb.python/py-objfile.c: Add global and static vars.
	* gdb.python/py-objfile.exp: Test new functions Objfile.
	  lookup_global_symbol and lookup_static_symbol.