gdb: add inferior parameter to architecture_changed observable

gdb: add inferior parameter to architecture_changed observable

This is to make it explicit which inferior's architecture just changed,
and that the callbacks should not assume it is the current inferior.

Update the only caller, pyuw_on_new_gdbarch, to add the parameter,
although it doesn't use it currently.

Change-Id: Ieb7f21377e4252cc6e7b1ce2cc812cd1a1840e0e
Reviewed-By: John Baldwin <jhb@FreeBSD.org>
Approved-By: Andrew Burgess <aburgess@redhat.com>