MN10300: Emulate single stepping in KGDB on MN10300

Emulate single stepping in KGDB on MN10300 by way of temporary breakpoint
insertion.  These breakpoints are never actually seen by KGDB, and will overlay
KGDB's own breakpoints.

The breakpoints are removed by switch_to() and reinstalled on switching back so
that if preemption occurs, the preempting task doesn't hit them (though it will
still hit KGDB's regular breakpoints).  If KGDB is reentered for any reason,
then the single step breakpoint is completely erased and must be set again by
the debugger.

We take advantage of the fact that KGDB will effectively halt all other CPUs
whilst this CPU is single-stepping to avoid SMP problems.

If the single-stepping task is preempted and killed without KGDB being
reinvoked, then the breakpoint(s) will be cleared and KGDB will be jumped back
into.

Signed-off-by: David Howells <dhowells@redhat.com>