selftests/bpf: Test ldsx with more complex cases

The following ldsx cases are tested:
- signed readonly map value
- read/write map value
- probed memory
- not-narrowed ctx field access
- narrowed ctx field access.

Without previous proper verifier/git handling, the test will fail.

If cpuv4 is not supported either by compiler or by jit,
the test will be skipped.

# ./test_progs -t ldsx_insn
#113/1 ldsx_insn/map_val and probed_memory:SKIP
#113/2 ldsx_insn/ctx_member_sign_ext:SKIP
#113/3 ldsx_insn/ctx_member_narrow_sign_ext:SKIP
#113 ldsx_insn:SKIP
Summary: 1/0 PASSED, 3 SKIPPED, 0 FAILED

Signed-off-by: Yonghong Song <yonghong.song@linux.dev>
Link: https://lore.kernel.org/r/20230728011336.3723434-1-yonghong.song@linux.dev
Signed-off-by: Alexei Starovoitov <ast@kernel.org>