This is a Preview release of Mono, so we use `centos8-preview` packages which
correspond to its MSBuild version requirements (16.10.1).
For osxcross, we need to build compiler-rt to be able to link the new Apple
Silicon support code. See #83 for details.
Includes a backport of https://github.com/tpoechtrager/osxcross/pull/284 to
simplify the install process.
Actual macOS arm64 builds will be added in a follow-up commit.
Rémi Verschelde