[POWERPC] spufs: don't (ab)use SCHED_IDLE

commit 4ef11014

 introduced a usage of SCHED_IDLE to detect when
a context is within spu_run.

Instead of SCHED_IDLE (which has other meaning), add a flag to
sched_flags to tell if a context should be running.

Signed-off-by: Jeremy Kerr <jk@ozlabs.org>