tools/llvm-bpf: update to 21.1.6

tools/llvm-bpf: update to 21.1.6

Release Notes:
- https://discourse.llvm.org/t/llvm-21-1-6-released
- https://discourse.llvm.org/t/llvm-21-1-5-released
- https://discourse.llvm.org/t/llvm-21-1-4-released
- https://discourse.llvm.org/t/llvm-21-1-3-released
- https://discourse.llvm.org/t/llvm-21-1-2-released
- https://discourse.llvm.org/t/llvm-21-1-1-released
- https://discourse.llvm.org/t/llvm-21-1-0-released
- https://discourse.llvm.org/t/llvm-20-1-7-released

Link: https://github.com/openwrt/openwrt/pull/20870
Signed-off-by: Nick Hainke <[email protected]>