build-manifest: Add `rust-mingw` also as extension for "pc-windows-gnu" hosts. This should enable `rustup component add --target aarch64-pc-windows-gnullvm rust-mingw` when running an `x86_64-pc-windows-gnullvm` toolchain (and vice-versa). Which itself enables proper cross-compiling of Windows ARM64 binaries on a Windows x64 host without using commercial MSVC. CC @mati865