GCC 10 doesn't build for any MinGW configuration

[kira-bruneau]

Describe the bug
mcfgthread is patched into GCC when building for MinGW, however an equivalent GCC 10 patch file doesn't exist. I tried looking for a GCC 10 patch on the master branch of https://github.com/lhmouse/MINGW-packages, but the mingw-w64-gcc-git folder that contained the patches no longer exists.

GCC9 is still currently the default on master, but this will break everything under MinGW the next time staging-next is merged to master. See #89793.

Support for mcfgthread was originally added in #73195.

To Reproduce
Steps to reproduce the behavior:

1. nix-build '<nixpkgs>' -A pkgsCross.mingwW64.buildPackages.gcc10

curl: (22) The requested URL returned error: 404 Not Found
error: cannot download 9000-gcc-10-branch-Added-mcf-thread-model-support-from-mcfgthread.patch from any mirror

Notify maintainers
@Ericson2314 @Synthetica9

Metadata

• system: "x86_64-linux"
• host os: Linux 5.10.2, NixOS, 21.03.git.258134b2335 (Okapi)
• multi-user?: yes
• sandbox: yes
• version: nix-env (Nix) 2.3.10
• channels(root): "nixos-21.03pre259798.84917aa00bf, nur"
• channels(kira): "home-manager"
• nixpkgs: /nix/var/nix/profiles/per-user/root/channels/nixos

Maintainer information:

# a list of nixpkgs attributes affected by the problem
attribute: pkgsCross.mingw32.buildPackages.gcc10, pkgsCross.mingwW64.buildPackages.gcc10
# a list of nixos modules affected by the problem
module:

[lhmouse]

It has been moved to https://github.com/lhmouse/MINGW-packages-dev due to upstream migration. Neither of these repos is stable; their master branches are constantly rebased on upstream and force-pushed.