m68k: add TIF_NOTIFY_RESUME and handle it.

TIF_NOTIFY_RESUME added (as bit 5).  That way nommu glue needs no changes at
all; mmu one needs just to replace jmi do_signal_return to jne do_signal_return
There we have flags shifted up, until bit 6 (SIGPENDING) is in MSBit; instead
of checking that MSBit is set (jmi) we check that MSBit or something below it
is set (jne); bits 0..4 are never set, so that's precisely "bit 6 or bit 5 is
set".

Usual handling of NOTIFY_RESUME/SIGPENDING is done in do_notify_resume(); glue
calls it instead of do_signal().

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

arch/m68k/include/asm/thread_info.h

@@ -71,6 +71,7 @@ static inline struct thread_info *current_thread_info(void)
  * bits 0-7 are tested at every exception exit
  * bits 8-15 are also tested at syscall exit
  */
+#define TIF_NOTIFY_RESUME	5	/* callback before returning to user */
 #define TIF_SIGPENDING		6	/* signal pending */
 #define TIF_NEED_RESCHED	7	/* rescheduling necessary */
 #define TIF_DELAYED_TRACE	14	/* single step a syscall */