ARMv7-M defines a "lockup" state that's entered in certain double
fault sequences which can't be recovered from without external help.
OpenOCD has previously ignored this.

Issue a diagnostic saying the chip has locked up, and force exit
from this state by halting the core.  It's not clear this is the
best way to handle lockup; but there should now be less confusion.

Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>