gdb: add interp::on_inferior_added method

gdb: add interp::on_inferior_added method

Same idea as previous patches, but for inferior_added.

mi_interp::init avoided using mi_inferior_added, since, as the comment
used to say, it would notify all MI interpreters.  Now, it's easy to
only notify the new interpreter, so it's possible to just call the
on_inferior_added method in mi_interp::init.

Change-Id: I0eddbd5367217d1c982516982089913019ef309f