llvm-jit: patch lddw helpers at compile time

[Officeyutong]

This PR updated llvm-jit so it will execute lddw helpers (https://docs.kernel.org/bpf/standardization/instruction-set.html#id33) at runtime and hardcodes generated values into LLVM IR, thus reducing runtime oveahead

Also set default implementation for map_by_fd and map_by_idx: returning its input value

• One exception is that map_val won't be patched for AOT compilations, since AOT-ed programs could be migrated among processed, and map_val (a memory address) may be invalided among processes