selftests/bpf: Add C tests for 16-byte returns in R0: R2

Add selftests that exercise a 16-byte return value passed in the R0:R2
register pair, written in C so that they depend on the compiler lowering
the register-pair return.

The R0:R2 convention is only emitted by LLVM 23 and newer, and a by-value
aggregate return does not compile at all before that, so the programs sit
behind a __clang_major__ guard. An older compiler builds the dummy test in
the #else branch instead, which keeps the object non-empty and says in its
description why nothing was exercised.

The kfunc tests are tagged __arch_x86_64/__arch_arm64 and skip elsewhere.
Those are the architectures whose JIT advertises
bpf_jit_supports_kfunc_ret_reg_pair(), which bpf_add_kfunc_call() requires
before it accepts a kfunc returning more than 8 bytes, and they are also
the only ones building the kfuncs.

Signed-off-by: Yonghong Song <yonghong.song@linux.dev>
Acked-by: Eduard Zingerman <eddyz87@gmail.com>
Link: https://patch.msgid.link/20260819055320.3300114-1-yonghong.song@linux.dev
Signed-off-by: Eduard Zingerman <eddyz87@gmail.com>

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