selftests/bpf: Add unit tests with __bpf_trap() kfunc

selftests/bpf: Add unit tests with __bpf_trap() kfunc

Add some inline-asm tests and C tests where __bpf_trap() or
__builtin_trap() is used in the code. The __builtin_trap()
test is guarded with llvm21 ([1]) since otherwise the compilation
failure will happen.

  [1] https://github.com/llvm/llvm-project/pull/131731

Signed-off-by: Yonghong Song <yonghong.song@linux.dev>
Link: https://lore.kernel.org/r/20250523205331.1291734-1-yonghong.song@linux.dev
Tested-by: Eduard Zingerman <eddyz87@gmail.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>