ETM cleanup

Various cleanups of ETM related code.

 - Saner error return paths
 - Simplify arm7_9 init ... no need for extra zeroing!
 - Shrink some lines
 - Tweak some diagnostics
 - Use shorter name for ETM struct type.
 - Don't exit()

and similar.  The diagnostics look forward to having
this ETM code work with more than just ARM7/ARM9.

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