tools/sched_ext: Receive misc updates from SCX repo

Receive misc tools/sched_ext updates from https://github.com/sched-ext/scx


to sync userspace bits.

- LSP macros to help language servers.

- bpf_cpumask_weight() declaration and cast_mask() helper.

- Cosmetic updates to scx_flatcg.bpf.c.

Signed-off-by: Tejun Heo <tj@kernel.org>