selftests/bpf: Add unit tests for new sign-extension load insns

selftests/bpf: Add unit tests for new sign-extension load insns

Add unit tests for new ldsx insns. The test includes sign-extension
with a single value or with a value range.

If cpuv4 is not supported due to
  (1) older compiler, e.g., less than clang version 18, or
  (2) test runner test_progs and test_progs-no_alu32 which tests
      cpu v2 and v3, or
  (3) non-x86_64 arch not supporting new insns in jit yet,
a dummy program is added with below output:
  #318/1   verifier_ldsx/cpuv4 is not supported by compiler or jit, use a dummy test:OK
  #318     verifier_ldsx:OK
to indicate the test passed with a dummy test instead of actually
testing cpuv4. I am using a dummy prog to avoid changing the
verifier testing infrastructure. Once clang 18 is widely available
and other architectures support cpuv4, at least for CI run,
the dummy program can be removed.

Signed-off-by: Yonghong Song <yonghong.song@linux.dev>
Link: https://lore.kernel.org/r/20230728011304.3719139-1-yonghong.song@linux.dev
Signed-off-by: Alexei Starovoitov <ast@kernel.org>