for powerpc.  When set,
we call tracehook_notify_resume() on the way to user mode.
This overloads do_signal() to do the work, but changes its
arguments to it has the TIF_* bits handy in a register and
drops the useless first argument that was always zero.

Signed-off-by: Roland McGrath <roland@redhat.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
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