This patch enables mapping host directories to guest directories by parsing
the `map_dir_list` argument in API `wasm_runtime_init_wasi` for libc-wasi. It
follows the format `<guest-path>::<host-path>`.
It also adds argument `--map-dir=<guest::host>` argument for `iwasm`
common line tool, and allows to add multiple mappings:
```bash
iwasm --map-dir=<guest-path1::host-path1> --map-dir=<guest-path2::host-path2> ...
```
Adapt API usage to new interfaces where applicable, including LLVM function
usage, obsoleted llvm::Optional type and removal of unavailable headers.
Know issues:
- AOT static PGO isn't enabled
- LLVM JIT may run failed due to llvm_orc_registerEHFrameSectionWrapper
isn't linked into iwasm
