[PATCH v3] m68k: improved switch stack handling

m68k version of Eric's 'improved switch stack handling' patch for alpha. 

The first two patches address m68k missing saving switch_stack on
those syscalls that can may call ptrace_stop(), and adding a full stack
frame in kernel threads.

The last patch adds a 'status' field to m68k thread_info struct, and
stores information about whether a syscall trace is in progress, and
a full stack frame has been saved, in that field. This information can
be used in arch/m68k/kernel/ptrace.c by get/put_reg() to guard against
accessing incorrect information on the stack (haven't got around to
that bit yet). I'm quite certain I haven't picked the most efficient
implementation here - suggestions to optimize most welcome!

Cheers,

   Michael