selftests/bpf: tests for __arg_untrusted void * global func params

selftests/bpf: tests for __arg_untrusted void * global func params

Check usage of __arg_untrusted parameters of primitive type:
- passing of {trusted, untrusted, map value, scalar value, values with
  variable offset} to untrusted `void *`, `char *` or enum is ok;
- varifier represents such parameters as rdonly_untrusted_mem(sz=0).

Acked-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
Signed-off-by: Eduard Zingerman <eddyz87@gmail.com>
Link: https://lore.kernel.org/r/20250704230354.1323244-9-eddyz87@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>