tools/sched_ext/include: Sync bpf_arena_common.bpf.h with scx repo

tools/sched_ext/include: Sync bpf_arena_common.bpf.h with scx repo

Sync the following changes from the scx repo:

- Guard __arena define with #ifndef to avoid redefinition when the
  attribute is already defined by another header.
- Add bpf_arena_reserve_pages() and bpf_arena_mapping_nr_pages() ksym
  declarations.
- Rename TEST to SCX_BPF_UNITTEST to avoid collision with generic TEST
  macros in other projects.

Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: Andrea Righi <arighi@nvidia.com>