selftests/bpf: Move arena-related headers into libarena

The BPF selftest headers include functionality that is
specific to arenas and is required by libarena. Keep libarena
self-contained by moving

selftests/bpf: Move arena-related headers into libarena

The BPF selftest headers include functionality that is
specific to arenas and is required by libarena. Keep libarena
self-contained by moving all functionality into its include/
directory. Also add libarena/include to the standard include
paths for the selftests to make the moved headers easy to
access by existing selftests.

Some functionality is required by libarena but not strictly
arena-related. We still move it to the libarena/include path,
which is an upgrade from directly accessing them from the
selftests/bpf directory using relative paths.

A new bpf_may_goto.h file is split off of bpf_experimental.h.
bpf_arena_spin_lock.h and bpf_arena_common.h are moved to
libarena/include. bpf_atomic.h is also moved to libarena
because it is necessary for arena spinlocks.

For bpf_arena_spin_lock.h, mark the spinlock state array as __weak
to define the spinlock state array in the header while also
being compatible with multi-compilation unit programs. While
we're at it, we remove unnecessary definitions from existing
test programs.

Signed-off-by: Emil Tsalapatis <emil@etsalapatis.com>
Link: https://lore.kernel.org/r/20260426190338.4615-4-emil@etsalapatis.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

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