selftests/bpf: Introduce __arena_global

While clang uses __attribute__((address_space(1))) both for defining
arena pointers and arena globals, GCC requires different syntax for
both. While __arena covers the first use case, introduce __arena_global
to cover the second one.

Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20240701234304.14336-11-iii@linux.ibm.com