selftests/bpf: Add selftest for attaching fexit to __noreturn functions

The reuslt:

  $ tools/testing/selftests/bpf/test_progs --name=fexit_noreturns
  #99/1    fexit_noreturns/noreturns:OK
  #99      fexit_noreturns:OK
  Summary: 1/1 PASSED, 0 SKIPPED, 0 FAILED

Signed-off-by: Yafang Shao <laoar.shao@gmail.com>
Link: https://lore.kernel.org/r/20250318114447.75484-3-laoar.shao@gmail.com


Signed-off-by: Alexei Starovoitov <ast@kernel.org>