sched_ext: Sync common and compat headers from the scx repo

Sync common.bpf.h, compat.bpf.h and compat.h with the scx repo, which
accumulated the following:

- __COMPAT_read_enum() can now recover 64-bit scx enum values from kernel
  BTF generated without BTF_KIND_ENUM64 support (pahole < 1.24 or
  --skip_encoding_btf_enum64, e.g. COS/GKE kernels), substituting values
  from the build-time vmlinux.h cross-checked against the low 32 bits the
  kernel does provide.

- is_migration_disabled() no longer assumes the BPF prolog always disables
  migration. Since 8e4f0b1ebcf2 ("bpf: use rcu_read_lock_dont_migrate() for
  trampoline.c") the prolog only does so under CONFIG_PREEMPT_RCU, so the
  old current-task test under-reported on v6.18+ !PREEMPT_RCU kernels. A
  runtime probe on bpf_scx_reg() handles older kernels with backported
  trampoline behavior.

- __COMPAT_scx_bpf_dsq_peek() is gated behind kernel v7.1 where 2f2ea7709266
  ("sched_ext: Use dsq->first_task instead of list_empty() in
  dispatch_enqueue() FIFO-tail") fixed the kfunc spuriously returning NULL
  on non-empty FIFO DSQs, and the new
  scx_bpf_reenqueue_local_from_anywhere() provides a callable-from-anywhere
  reenqueue which prefers the generic scx_bpf_dsq_reenq(). Both were first
  posted by Gavin Guo and Changwoo Min and are picked up here with the
  review feedback folded in.

- __COMPAT_scx_bpf_cpu_curr() and the scx_bpf_cpu_rq() declaration are
  restored. Schedulers built from these headers still run on pre-v6.18
  kernels where scx_bpf_cpu_curr() does not resolve and the scx_bpf_cpu_rq()
  fallback still exists.

- scx_clock_task() and scx_clock_pelt() document their stale-read behavior
  for remote idle CPUs under NO_HZ_IDLE.

Link: https://lore.kernel.org/all/20260817143126.562923-1-changwoo@igalia.com
Signed-off-by: Tejun Heo <tj@kernel.org>

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