Cortex-A8: mode support

Cortex-A8: mode support

We *should* be able to read and write registers in any core mode,
instead of being stuck with whatever mode the core was when we
entered debug state.  This patch makes them work.

Note that the current restore_context() only handles the current
mode; writing to other-mode registers is a NOP without a followup
patch fixing that.  Also, that SPSR access needed some bugfixes;
it was confused with CPSR.

Secure monitor mode also seems dubious; there's probably more to
be done before that's sufficiently understood by the debugger.

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