Generalize the core of watchpoint setup so that it can handle
breakpoints too.  Create breakpoint add/remove routines which
will use that, and hook them up to target types which don't
provide their own breakpoint support (nothing, yet).

This suffices for hardware-only breakpoint support.  The ARM11
code will be able to switch over to this without much trouble,
since it doesn't yet handle software breakpoints.  Switching
Cortex-A8 will be a bit more involved.

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