Merge branch 'selftests-bpf-libarena-add-initial-data-structures'

Emil Tsalapatis says:

====================
selftests/bpf: libarena: Add initial data structures

Add two new data structures to libarena. These data structures initially
resided in the sched-ext repo (https://github.com/sched-ext/scx) and
have been adapted to the internal libarena build system. The data
structures are:

- Red black tree: Fundamental tree data structure that can also serve
  as a base for more domain-specific data structures.

- Lev-Chase deque: Queue data structure that allows efficient work
  stealing, useful in scheduling scenarios.

The data structures are accompanied by selftests that are automatically
discovered by the existing libarena test_progs selftest and incorporated
in the CI.

CHANGELOG
=========

v3 -> v4 (https://lore.kernel.org/bpf/20260604235016.20856-1-emil@etsalapatis.com/)
- Turn off load_acquire/store_relesase - dependent selftests for s390 (CI)
- Various style/non-functional nits (AI)

v2 -> v3 (https://lore.kernel.org/bpf/20260603182727.3922-1-emil@etsalapatis.com/)

- Add workaround to handle LLVM 21 and GCC 15 assignment-to-memset promotions
  that are causing verification failures for arena programs (CI)
- Incorporate Sashiko feedback for cleanup edge cases (Sashiko)
- Simplify some of the ordering semantics in spmc

v1 -> v2 (https://lore.kernel.org/bpf/20260511214100.9487-1-emil@etsalapatis.com/):

- Rename tests from st_ to test_ (Alexei)
- Removed the freelist caches from the rbtrees, previously used to defer freeing (Alexei)
- Moved the type and function definitions to use the __arena identifier
- Removed the typecasts during function return and directly return __arena
  pointers (Alexei)
- Renamed queues to spmc queues to abstract away the algorithm (Alexei)
- Adjusted the memory barriers in the spmc queue
- Added multithreaded testing harness for libarena programs (Alexei)
- Added parallel selftest for queues (Alexei)
- Split least upper bound and exact find operations back into separate
  functions to prevent RB_DUPLICATE-related bug (AI)
====================

Link: https://patch.msgid.link/20260605222020.5231-1-emil@etsalapatis.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>