selftests/bpf: Test kfuncs returning arena pointers by value

Cover the by-value struct returns a kfunc may now make: two arena
pointers filling R0:R2, and an arena pointer beside a scalar. Two further
cases drop the tag from one member of a struct and one arm of a union,
and stay rejected naming that member, so what decides is the tag rather
than the member being a pointer. The existing cases for a struct and a
nested struct carrying a plain pointer stay rejected as well.

These cases call the kfuncs from C, so the compiler lowers the by-value
return itself, and a struct or union only lands in R0:R2 with the LLVM 23
BPF ABI. An older clang, and gcc, return it through a hidden pointer in
R1 instead, which shifts the arguments along and fails verification. The
file is therefore guarded on LLVM 23, falling back to a dummy test.

Signed-off-by: Yonghong Song <yonghong.song@linux.dev>
Link: https://lore.kernel.org/r/20260829061610.1699950-1-yonghong.song@linux.dev
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

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