selftests/bpf: Add union argument tests using fexit programs

selftests/bpf: Add union argument tests using fexit programs

Add test coverage for union argument support using fexit programs:

* 8B union argument - verify that the verifier accepts it and that fexit
  programs can trace such functions.
* 16B union argument - verify that the verifier accepts it and that
  fexit programs can access the argument, which is passed using two
  registers.

Signed-off-by: Leon Hwang <leon.hwang@linux.dev>
Link: https://lore.kernel.org/r/20250919044110.23729-3-leon.hwang@linux.dev
Signed-off-by: Alexei Starovoitov <ast@kernel.org>