gdb/python: add void_type () method to gdb.Architecture object

gdb/python: add void_type () method to gdb.Architecture object

This commit adds a new method to Python architecture objects that
returns a void type for that architecture.

This will be useful later to create types for function symbols created
using Python extension code.

Reviewed-By: Eli Zaretskii <eliz@gnu.org>
Approved-By: Andrew Burgess <aburgess@redhat.com>