selftests/bpf: Add tests for >8 byte return value and 128-bit arguments

The BPF trampoline preserves only 8 bytes of the target's return value
(R0), so attaching an fexit/fmod_ret/fsession program to a function that
returns a >8 byte value is now rejected by the verifier. Add a bpf_testmod
function returning __int128 and an fexit program that targets it. The
program is expected to fail to load with the "with a >8 byte return value
is not supported for this attach type" message.

A 128-bit __int128 argument is passed in a register pair and occupies two
trampoline context slots. Add a bpf_testmod function taking a leading
__int128 argument followed by an int and a long, and an fexit program that
reads those two trailing arguments and the return value, verifying that the
trampoline reserves enough stack for the 128-bit argument and places the
following arguments and the return value at the right context slots.

__int128 is only available on 64-bit targets (where the compiler defines
__SIZEOF_INT128__). The argument test additionally depends on the calling
convention: x86_64 and arm64 pass an __int128 in a register pair as the
trampoline expects, while other architectures pass it differently (e.g.
s390x passes larger arguments by reference), so that subtest runs only on
x86_64 and arm64 and is skipped elsewhere.

Signed-off-by: Yonghong Song <yonghong.song@linux.dev>
Acked-by: Leon Hwang <leon.hwang@linux.dev>
Link: https://lore.kernel.org/bpf/20260729050209.2587581-1-yonghong.song@linux.dev
Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>

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