selftests/bpf: arg tracking for imprecise/multi-offset BPF_ST/STX

selftests/bpf: arg tracking for imprecise/multi-offset BPF_ST/STX

Add test cases for clear_stack_for_all_offs and dst_is_local_fp
handling of multi-offset and ARG_IMPRECISE stack pointers:

- st_imm_join_with_multi_off: BPF_ST through multi-offset dst should
  join at_stack with none instead of overwriting both candidate slots.
- st_imm_join_with_imprecise_off: BPF_ST through offset-imprecise dst
  should join at_stack with none instead of clearing all slots.
- st_imm_join_with_single_off: a canary checking that BPF_ST with a
  known offset overwrites slot instead of joining.
- imprecise_dst_spill_join: BPF_STX through ARG_IMPRECISE dst should
  be recognized as a local spill and join at_stack with the written
  value.

Signed-off-by: Eduard Zingerman <eddyz87@gmail.com>
Link: https://lore.kernel.org/r/20260413-stacklive-fixes-v2-2-398e126e5cf3@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>