sched: treat sync waker CPUs with 1 task as idle

When a CPU with one task performs a sync wakeup, its
one task is expected to sleep immediately so this CPU
should be treated as idle for the purposes of CPU selection
for the waking task.

This is only done when idle CPUs are the preferred targets
for non-small task wakeups. When prefer_idle is 0, the
CPU is left as non-idle in the selection logic so it is still
a preferred candidate for the sync wakeup.

Change-Id: I65c6535169293e8ba0c37fb5e88aec336338f7d7
Signed-off-by: Steve Muckle <smuckle@codeaurora.org>