mn10300: Use common threadinfo allocator

Let the core code allocate and handle the kgdb cleanup with the
arch_release_thread_info() function.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: David Howells <dhowells@redhat.com>
Link: http://lkml.kernel.org/r/20120505150141.996582377@linutronix.de