selftests/bpf: Verify zext_dst annotations for various instructions

Includes the following test cases:

- a test showing that zero extension flags do not propagate through
  state pruning in the unpatched kernel.
- a 32-bit subregister consumed by MOV32 and ALU32 operations
  (never zext'ed);
- a 64-bit MOV (never zext'ed);
- a narrow (32-bit) BPF_LDX load whose result is read as 64-bit;
- 32-bit atomic fetch_add and cmpxchg whose result is read as 64-bit;
- a CFG case where a 32-bit definition's upper half is used only on one
  of two branches;
- no zext for dead registers;
- LD_ABS defines only lower 32 bits, hence needs zext when the result
  is used as 64-bits;
- helper, kfunc and subprogram parameters are considered to use full
  64 bits;
- a 32-bit subregister consumed by JMP32 (X/K) operations;
- a 32-bit subregister consumed by JMP (X/K) operations;
- a 64-bit register consumed by both JMP and JMP32 operations
  (never zext'ed);
- ALU64 and address space cast operations on arena pointers;
- memory loads using BPF_PROBE_MEM instructions.

Signed-off-by: Eduard Zingerman <eddyz87@gmail.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20260807-static-zext-v4-7-b6c270013c77@gmail.com

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