Add a WebAssembly back end

A WebAssembly back end has been discussed and attempted before (#14976 (closed), closed; https://github.com/tweag/asterius, incompatible with GHC's run-time system; https://webghc.github.io/, dormant), but one has never been truly incorporated into GHC. We propose to do it by adapting GHC's native code generator and GHC's run-time system.

The attempt we know best is Asterius, but Asterius currently relies on GHC's 8.10 branch, and instead of GHC's run-time system, it uses a custom run-time system hand-written in JavaScript.

• Wiki page on goals for the project: https://gitlab.haskell.org/ghc/ghc/-/wikis/WebAssembly-goals

• Wiki page with status, plans, and FAQ: https://gitlab.haskell.org/ghc/ghc/-/wikis/WebAssembly-backend

Things to do, to be refined as progress is made:

• WebAssembly native code generator

  • Add wrapper for doing dominator analysis on CmmGraph (related to #20726)
  • Add key graph-algorithm logic from FGL (#21259)
  • Add "node splitter" to convert irreducible control-flow graphs
  • Add representaiton of WebAssembly control flow
  • Add translation from CmmGraph to WebAssembly control flow
  • Add representation of WebAssembly straight-line code
  • Add representation of WebAssembly relocatable object/asm files
  • Add codeOutput function to emit WebAssembly from CmmGraph
  • Add post-HSC pipeline to support assembly and linking
  • Add WebAssembly back end to compiler driver

• WebAssembly runtime independent of JavaScript:

  • Update build system to add a new platform (Wasi/Wasm32) to the existing POSIX and Win32 platforms
  • Build RTS and all boot libs for Wasi/Wasm32 without POSIX dependencies

• JavaScript interoperation:

  • Refactor RTS schedule function to simplify future updates (#20968)
  • Add support for GHC/RTS inside JavaScript event loop, as per operational semantics
  • Reach agreement with GHCJS on support for JavaScript FFI