Add new baseline windows-gnu toolchains

[mati865]

CC rust-lang/compiler-team#993 rust-lang/rust#157842

[marcoieni]

Is the repository https://github.com/niXman/mingw-builds-binaries the official way of retrieving these artifacts?
The repo seems to belong to a single github user (not an organization), and the github actions are not up-to-date. Eg it uses actions/checkout@v2.

[mati865]

Mingw-w64 provides no official prebuilt artifacts, without other tools like compiler, assember and linker they wouldn't be useful anyway. They only provide list of prebuilt toolchains that are not maintained by them: https://www.mingw-w64.org/downloads/

NiXman has been maintaining these builds since 2012 and all previous Rust releases relied on them. I know this might not be very convincing argument, but this is by a mile easiest way to pin specific version on the CI.