selftests/bpf: libarena: Add "arena" BTF type tag to __arena qualifier

The arena qualifier currently designates its associated type
as belonging to address space 1. This property affects code
generation, but is not reflected in the BTF information of
the function.

This lack of information at the BTF level prevents us from
returning arena pointers from global subprograms. Subprogs
cannot return any data structure more complex than a scalar,
so pointers to structs are rejected as a return type. We
have no way of marking the return type as a pointer to an
arena, which is safe provided the two subprogs have the same
arena.

Expand the __arena qualifier to also attach a BTF type tag
to the type. This lets us determine whether a variable belongs
to an arena from its type alone through BTF parsing.

Signed-off-by: Emil Tsalapatis <emil@etsalapatis.com>
Link: https://lore.kernel.org/r/20260602004120.17087-2-emil@etsalapatis.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

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