sched/fair: select the most energy-efficient CPU candidate on wake-up

The current implementation of the energy-aware wake-up path relies on
find_best_target() to select an ordered list of CPU candidates for task
placement. The first candidate of the list saving energy is then chosen,
disregarding all the others to avoid the overhead of an expensive
energy_diff.

With the recent refactoring of select_energy_cpu_idx(), the cost of
exploring multiple CPUs has been reduced, hence offering the opportunity
to select the most energy-efficient candidate at a lower cost. This commit
seizes this opportunity by allowing to change select_energy_cpu_idx()'s
behaviour as to ignore the order of CPUs returned by find_best_target()
and to pick the best candidate energy-wise.

As this functionality is still considered as experimental, it is hidden
behind a sched_feature named FBT_STRICT_ORDER (like the equivalent
feature in Android 4.14) which defaults to true, hence keeping the
current behaviour by default.

Change-Id: I0cb833bfec1a4a053eddaff1652c0b6cad554f97
Suggested-by: Patrick Bellasi <patrick.bellasi@arm.com>
Suggested-by: Chris Redpath <chris.redpath@arm.com>
Signed-off-by: Quentin Perret <quentin.perret@arm.com>