gdb: scope down registers_changed call in inferior::set_arch

gdb: scope down registers_changed call in inferior::set_arch

inferior::set_arch calls registers_changed, which invalidates all
regcaches.  It would be enough to invalidate only regcaches of threads
belonging to this inferior.  Call registers_changed_ptid instead, with
the proper process target / ptid.  If the inferior does not have a
process target, there should be no regcaches for that inferior, so no
need to invalidate anything.

Change-Id: Id8b5500acb7f373b01a534f16d3a7d028dc0d882
Reviewed-By: John Baldwin <jhb@FreeBSD.org>
Approved-By: Andrew Burgess <aburgess@redhat.com>