Add Cirrus CI Mac M1 build

[elopez]

This adds automated builds using Cirrus CI, which offers M1 runners. The build process performed is similar to the current GH Actions Mac CI, which uses Stack. The main process changes include:

• brew install: add libtool haskell-stack hpack which are not available by default.
• --extra-include-dirs=/opt/homebrew/include --extra-lib-dirs=/opt/homebrew/lib: Fixes failure to link with gmp.
• --resolver=ghc-8.10.7, --compiler=ghc-8.10.7: GHC 8.10.4 is not available on M1. 8.10.5 hanged during build 😞 8.10.7 works fine 🎉 not needed anymore with lts-20.05
• --with-hpack hpack to fix support for GHC2021: https://github.com/commercialhaskell/stack/issues/5739

Enabling the CI requires installing a GitHub Application to the org/repository.

Example build: https://cirrus-ci.com/task/6248730202996736