Allow Rust to build against ports LLVM.
- Build issues in PR/223300,223342 are no longer reproducible with Rust 1.56 and LLVM 13.
- https://github.com/rust-lang/rust/issues/46181 is fixed.
- Upstream tracks upstream LLVM "pretty closely" .
llvm-config in cargo.toml implies linking to LLVM libraries, -Lnative is passed for linking to proceed.