sparc: We need to implement arch_ptrace_stop().

In order to always provide fully synchronized state to the debugger,
we might need to do a synchronize_user_stack().

A pair of hooks, arch_ptrace_stop_needed() and arch_ptrace_stop(),
exist to handle this kind of situation.  It was created for
the sake of IA64.

Use them, to flush the kernel side cached register windows
to the user stack, when necessary.

Signed-off-by: David S. Miller <davem@davemloft.net>