Rename "struct armv4_5_common_s" as "struct arm".  It needs
a bit more work to be properly generic, and to move out of
this header, but it's the best start we have on that today.

Add and initialize an optional ETM pointer, since that will
be the first thing that gets generalized.

The intent being:  all ARMs should eventually derive from
this "struct arm", so they can reuse the current ETM logic.
(And later, more.)  Currently the ARM cores that *don't* so
derive are only ARMv7-M (and thus Cortex-M3) and ARM11.

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