message: Add instructions to build libcore
diff: cargo build || exit 1
  rustc -Zcodegen-backend=$(pwd)/target/debug/librustc_codegen_cranelift.so example.rs --crate-type lib -Og
  -rustc -Zcodegen-backend=$(pwd)/target/debug/librustc_codegen_cranelift.so ../rust/src/libcore/lib.rs --crate-type lib -Og
  +rustc -Zcodegen-backend=$(pwd)/target/debug/librustc_codegen_cranelift.so ./rust/src/libcore/lib.rs --crate-type lib -Og
  