Support linking with compiled C files

[hsyl20]

Support for linking with compiled C files would be very useful:

• some parts of the RTS are written in C (e.g. STM.c) and could be used directly without having to be manually converted into Asterius EDSL
• some packages use external c-sources

Options:

1. write a c-to-cmm compiler

   • Pro: integrate well into the existing Asterius pipeline
   • Con: hard to get right
   • Con: lot of work to support the full C language

2. Use LLVM (Clang) to compile into Wasm

   • Doc: https://github.com/WebAssembly/tool-conventions/blob/master/Linking.md
   • Pro: already working toolchain for C, C++, etc.
   • Con: produce objects that may be difficult to use (link with, etc.)

3. Compile LLVM-IR into Cmm?

   • Pro: already working toolchain for C, C++, etc.
   • Pro: Integrate well into the existing Asterius pipeline

[hsyl20]

Note that it would be good to share the approach we use with GHCJS.

[lemmih]

As the author of a few packages with c-sources, I'm interested in seeing how this turns out.