Add gdb.Architecture.integer_type Python function

Add gdb.Architecture.integer_type Python function

This adds a new Python function, gdb.Architecture.integer_type, which
can be used to look up an integer type of a given size and
signed-ness.  This is useful to avoid dependency on debuginfo when a
particular integer type would be useful.

v2 moves this to be a method on gdb.Architecture and addresses other
review comments.