[POWERPC] spufs: Fix SPE single-step mode

Currently we only catch debug events through the 0x3fff status;
spufs_run_spu doesn't handle single-step SPE events.

This change adds a handler for conditions where the SPE is stopped due
to single-step-mode.

Signed-off-by: Jeremy Kerr <jk@ozlabs.org>
Signed-off-by: Paul Mackerras <paulus@samba.org>