WebAssembly backend

This MR is now the launchpad for GHC's initial wasm32-wasi target support for 9.6. https://gitlab.haskell.org/ghc/ghc/-/wikis/WebAssembly-backend provides more context on this work.

Status update 11/06/2022:

It's ready for merge. The branch is based on !9181 (merged) which should be merged in first.

Status update 11/03/2022:

The commit history has been cleaned up completely, with commit messages and also notes when necessary. Once CI is green (awaiting some submodule bumps) it's ready for merge.

Status update 10/27/2022:

• Removed the KnownFuncs.hs hack in favor of import CLOSURE annotations in the hand-written Cmm files. We can now rely on isCFunctionLabel as the single source of truth for deciding symbol kind in the NCG.

Status update 10/24/2022:

• First pass at git history rewrite, also rebasing on master again, adding fix for !3012 (closed)

The WIP commits have all been replaced by git commits with descriptions. Expecting to wrap this up tomorrow morning, with clearer commit messages and notes.

Status update 10/21/2022:

The changeset in rts, libraries and codegen is now fully stablized.