selftests/bpf: Add a C++ prog with exceptions

  static __noinline int inner(int len)
  {
          rcu_guard guard;

          if (len > 100)
                  throw 1;
          return len;
  }

  static __noinline int middle(int len)
  {
          preempt_guard guard;

          return inner(len) + 1;
  }

  SEC("tc") int catches(struct __sk_buff *skb)
  {
          try {
                  return middle(skb->len);
          } catch (...) {
                  return 42;
          }
  }

The exception passes through two frames. The destructors call
bpf_rcu_read_unlock() and bpf_preempt_enable(). The verifier insists on
both, and the test checks that they ran in that order.
In the second prog nobody catches. The destructors run and the prog
returns 0.

All that is left of the C++ runtime is four one line functions.
__cxa_throw() calls _Unwind_RaiseException(), which llvm lowers.

progs/*.cpp are built like progs/*.rs: by clang only and without skeletons.
vmlinux.h and bpf_helpers.h are not C++, so the prog declares what it
needs.

clang that lowers unwinding [1] defines __BPF_FEATURE_UNWIND. Without it
the object has no progs and the test is skipped.

[1] https://github.com/llvm/llvm-project/pull/233649

Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://patch.msgid.link/20261020181320.1284798-6-alexei.starovoitov@gmail.com

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