ETM: add "etm trigger_debug" command

ETM: add "etm trigger_debug" command

In conjunction with manual register setup, this lets the ETM trigger
cause entry to debug state.   It should make it easier to test and
bugfix the ETM code, by enabling non-trace usage and isolating bugs
specific to thef ETM support.  (One current issue being that trace
data collection using the ETB doesn't yet behave.)

For example, many ARM9 cores with an ETM should be able to implement
four more (simple) breakpoints and two more (simple) watchpoints than
the EmbeddedICE supports.  Or, they should be able to support complex
breakpoints, incorporating ETM sequencer, counters, and/or subroutine
entry/exit criteria int criteria used to trigger debug entry.

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