What to do with gmp wasm fixes

[Cheng Shao]

Hi all,

The ghc wasm32-wasi build needs to patch gmp. Currently, our working
branch uses a fork of gmp-tarballs that includes the patch into the
tarball, but at some point we need to upstream it. What's the best way
to add these fixes?

- Send a PR to gmp-tarballs, including our patch (doesn't alter
behavior on native archs) and the updated tarball build script
- Don't touch gmp-tarballs, use "system" gmp, so the wasm32-wasi gmp
build process is decoupled from ghc build
- Give up gmp completely, only support native bignum for wasm32.

Cheers.
Cheng