selftests/bpf: Test by-value __int128 kfunc arguments at an odd slot

This follows up the series "bpf: Support by-value struct and __int128
arguments" [1]. The __int128 kfunc tests it added pass the __int128 at
an even argument slot, after two u64s, or past the argument registers,
where it lands on an even arm64 stack slot. So neither the AAPCS64 rule
that rounds the register number up to an even one for an argument
aligned to 16 bytes, nor the same rule for a stack slot, is exercised.
Each leaves a hole the arm64 JIT has to open by moving the __int128 and
every argument after it up a slot.

Add three kfuncs. Like the earlier ones, each weighs its arguments by
argument slot, so that two eightbytes landing in each other's slots are
caught as well as one landing in a hole:

  - bpf_kfunc_call_test_i128_arg_odd(): the __int128 at an odd register
    slot.
  - bpf_kfunc_call_test_i128_arg_odd_many(): the same, with arguments
    after it that arm64 shifts onto its stack, and a second __int128 at
    an odd arm64 stack slot.
  - bpf_kfunc_call_test_i128_arg_stack_hole(): a struct holding an
    __int128 at an odd stack slot on both x86-64 and arm64, after x86-64
    gives R9 back to an argument following a 16-byte struct.

x86-64 has no register alignment rule and places the first two like the
BPF convention, so they check that its JIT leaves them where they are.
The last two need more than the five BPF argument registers and are only
built with a compiler that supports stack arguments.

On arm64 the first two kfuncs get BTF only with the pahole patch
"dwarf_loader: Skip the argument register the arm64 ABI leaves as a hole"
[2], in pahole master after v1.32. An older pahole finds the argument
after the __int128 in an unexpected register, x4 rather than x3, and
leaves the function out of BTF, so the tests fail to load there.

Signed-off-by: Yonghong Song <yonghong.song@linux.dev>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20260912195156.980886-1-yonghong.song@linux.dev/ [1]
Link: https://lore.kernel.org/dwarves/20260917164043.1577508-1-yonghong.song@linux.dev/ [2]
Link: https://patch.msgid.link/20261016183054.1134162-1-yonghong.song@linux.dev

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