[NativeAOT-LLVM] Separate our WASM Jit from the upstream WASM Jit

We need to rename stuff as the upstream work progresses s. t. they don't collide on next merge:
1) `wasmjit` -> `llvmjit`.
2) `TARGET_WASM` (in the Jit) -> `TARGET_LLVM` / take upstream as-is where appropriate.