gdb/python: Add gdb.InferiorThread.__dict__ attribute

gdb/python: Add gdb.InferiorThread.__dict__ attribute

The gdb.Objfile, gdb.Progspace, gdb.Type, and gdb.Inferior Python
types already have a __dict__ attribute, which allows users to create
user defined attributes within the objects.  This is useful if the
user wants to cache information within an object.

This commit adds the same functionality to the gdb.InferiorThread
type.

After this commit there is a new gdb.InferiorThread.__dict__
attribute, which is a dictionary.  A user can, for example, do this:

  (gdb) pi
  >>> t = gdb.selected_thread()
  >>> t._user_attribute = 123
  >>> t._user_attribute
  123
  >>>

There's a new test included.

Reviewed-By: Eli Zaretskii <eliz@gnu.org>
Approved-By: Tom Tromey <tom@tromey.com>