selftests/bpf: Test global functions returning arena pointers by value

Cover the by-value struct returns a global function may now make: two
arena pointers filling R0:R2, an arena pointer beside a scalar, an array
of them, and an eight byte struct returned in R0 alone. The existing
cases for a struct and a union carrying a plain pointer stay rejected.

check_arena_struct_ret() also stores through both halves of the returned
pair and reads them back, so the test covers the returned pointers still
being usable as arena pointers rather than only the program verifying.

A toolchain guard goes into each file, and they differ because the
constructs do.

aggregate_ret_func.c uses "#if defined(__clang__)". Those cases are
__naked, so the function body is the inline asm and nothing else: the
clang compiler never lowers the return, and the 16 byte return type
reaches the verifier only through BTF. The minimum checked is LLVM 21.
gcc is excluded because it returns a by-value struct through a hidden
pointer and emits the 'r0 = r1' returning it after the __naked body's
exit, leaving the subprogram falling through.

verifier_arena.c uses "#if defined(__clang_major__) && __clang_major__ >=
23". arena_word_pair() returns the struct from C, so the compiler lowers
the return itself, and a 16 byte return only lands in R0:R2 with the
LLVM 23 BPF ABI. An older clang does not fall back to anything here, it
rejects the function with "aggregate returns are not supported", so the
floor is what lets the file build at all.

Signed-off-by: Yonghong Song <yonghong.song@linux.dev>
Link: https://lore.kernel.org/r/20260829061605.1698601-1-yonghong.song@linux.dev
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

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