tools/sched_ext: Receive updates from SCX repo

tools/sched_ext: Receive updates from SCX repo

Receive tools/sched_ext updates form https://github.com/sched-ext/scx to
sync userspace bits:

 - basic BPF arena allocator abstractions,

 - additional process flags definitions,

 - fixed is_migration_disabled() helper,

 - separate out user_exit_info BPF and user space code.

This also fixes the following warning when building the selftests:

 tools/sched_ext/include/scx/common.bpf.h:550:9: warning: 'likely' macro redefined [-Wmacro-redefined]
  550 | #define likely(x) __builtin_expect(!!(x), 1)
      |         ^

Co-developed-by: Cheng-Yang Chou <yphbchou0911@gmail.com>
Signed-off-by: Andrea Righi <arighi@nvidia.com>
Signed-off-by: Tejun Heo <tj@kernel.org>