selftests/bpf: Add tests for by-value kfunc arguments

Add kfuncs taking a 16-byte struct and an __int128 by value, with
combinations of <= 8 byte arguments and '> 8 && <= 16' byte arguments.
Each kfunc weighs its parameters by argument slot, the first by one, the
second by two and so on, and every test checks the value it returns. A
plain sum would be the same whichever slot each value reached, so an
argument that lands in the wrong one, or a 16-byte argument whose halves
arrive the other way round, would pass quietly; a weighted one differs.

An __int128 takes two argument slots. One test passes it in registers
and one past them, where both conventions pad the stack to align it
although the BPF convention does not, so both JITs move it up an
eightbyte.

Two more cover a rejection. An aggregate holding a pointer is refused
everywhere, and in arena_kfunc.c a two-slot struct pushes an arena
pointer past the argument registers, which is refused too. An aggregate
too large to pass by value is already covered in aggregate_arg_func.c.

test_stack_arg_big() in stack_arg_fail.c passed a 16-byte struct as the
sixth argument and asserted the unrecognized stack argument type it used
to be reported as. The JIT places that argument now, so the test is
removed.

Signed-off-by: Yonghong Song <yonghong.song@linux.dev>
Link: https://lore.kernel.org/r/20260912195313.992803-1-yonghong.song@linux.dev
Signed-off-by: Alexei Starovoitov <ast@kernel.org>