sched/debug: Print the local group's asym_prefer_cpu

Add a file to read local group's "asym_prefer_cpu" from debugfs. This
information was useful when debugging issues where "asym_prefer_cpu" was
incorrectly set to a CPU with a lower asym priority.

Signed-off-by: K Prateek Nayak <kprateek.nayak@amd.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lore.kernel.org/r/20250409053446.23367-5-kprateek.nayak@amd.com