ARM11: use standard single step simulation

ARM11: use standard single step simulation

The previous stuff was needed because the ARM11 code wasn't using
the standard ARM base type and register access ... but now those
mechanisms work, so we can switch out that special-purpose glue.

This should resolve all the "FIXME -- handle Thumb single stepping"
comments too, and properly handle the processor's mode.  (Modulo
the issue that this code doesn't yet handle two-byte breakpoints.)

Clarify the comments about the the hardware single stepping.  When
we eventually share breakpoint code with Cortex-A8, we can just make
that be the default on cores which support it.  We may still want an
override command, not just to facilitate testing but to cope with
"instruction address mismatch" not quite being true single-step.

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