kgdb: allow for cpu switch when single stepping

The kgdb core should not assume that a single step operation of a
kernel thread will complete on the same CPU.  The single step flag is
set at the "thread" level and it is possible in a multi cpu system
that a kernel thread can get scheduled on another cpu the next time it
is run.

As a further safety net in case a slave cpu is hung, the debug master
cpu will try 100 times before giving up and assuming control of the
slave cpus is no longer possible.  It is more useful to be able to get
some information out of kgdb instead of spinning forever.

Signed-off-by: Jason Wessel <jason.wessel@windriver.com>