mk: Add the ability to depend on native LLVM tools

mk: Add the ability to depend on native LLVM tools

The compiler will require that `llvm-ar.exe` be available for MSVC-targeting
builds (more comments on this soon), so this commit adds support for targets to
depend on LLVM tools. The `core` library for MSVC depends on `llvm-ar.exe` which
will be copied into place for the target before the compiler starts to run.

Note that these targets all depend on `llvm-config.exe` to ensure that they're
built before they're attempted to be copied.