selftests/bpf: test cases for callchain sensitive live stack tracking

selftests/bpf: test cases for callchain sensitive live stack tracking

- simple propagation of read/write marks;
- joining read/write marks from conditional branches;
- avoid must_write marks in when same instruction accesses different
  stack offsets on different execution paths;
- avoid must_write marks in case same instruction accesses stack
  and non-stack pointers on different execution paths;
- read/write marks propagation to outer stack frame;
- independent read marks for different callchains ending with the same
  function;
- bpf_calls_callback() dependent logic in
  liveness.c:bpf_stack_slot_alive().

Signed-off-by: Eduard Zingerman <eddyz87@gmail.com>
Link: https://lore.kernel.org/r/20250918-callchain-sensitive-liveness-v3-12-c3cd27bacc60@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>