ARM: keep a handle to the PC

ARM: keep a handle to the PC

Keep a handle to the PC in "struct arm", and use it.
This register is used a fair amount, so this is a net
minor code shrink (other than some line length fixes),
but mostly it's to make things more readable.

For XScale, fix a dodgy sequence while stepping.  It
was initializing a variable to a non-NULL value, then
updating it to handle the step-over-active-breakpoint
case, and then later testing for non-NULL to see if
it should reverse that step-over-active logic.  It
should have done like ARM7/ARM9 does: init to NULL.

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