ARM: add a default full_context() method

ARM: add a default full_context() method

If the core doesn't provide an optimized version of this
method, provide one without core-specific optimizations.
Use this to make Cortex-A8 support the "arm reg" command.

Related: make the two register access methods properly static,
have the "set" log a "not halted" error too, and make sure
that the "valid" flag is set on successful reads.

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