mn10300: signal stack fix

This fixes a signal stack handling problem in the MN10300 arch.  When new
threads are cloned with CLONE_VM, they don't inherit the alternate signal
stack.  They do share the signal flags, though.  When deciding whether to
use an alternate stack, the arch code needs to check to make sure the task
struct contains a valid alternate stack.  This patch fixes the MN10300
arch by using the sas_ss_flags() test provided by sched.h rather than the
on_sig_stack() test which is insufficient by itself.

Signed-off-by: Mark Salter <msalter@redhat.com>
Signed-off-by: David Howells <dhowells@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>