qt512: fix darwin patches

[ttuegel]

Motivation for this change

The patches for Qt 5.12 on Darwin no longer apply. Although I cannot test the build on macOS, I can make the Darwin patches apply on all platforms because the patches target only platform-specific files.

See also: #107483 (comment)

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS linux)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Determined the impact on package closure size (by running nix path-info -S before and after)
• Ensured that relevant documentation is up to date
• Fits CONTRIBUTING.md.

[prusnak]

I confirmed the patches do not apply on master when run on darwin (i.e. we need this PR to fix the build failure on master).
I confirmed the patches do apply in a61f20e when run on darwin and qt512 starts to build on darwin.
I have not tested the full build on darwin.

[ttuegel]

I'm still waiting on nixpkgs-review to verify that all the dependent packages build.

[r-burns]

Have not tested all deps, but this unblocks qtmultimedia 👍

[ttuegel]

I didn't find any new failures in the dependent packages.

[SuperSandro2000]

I will let you know if I see any other darwin related failures in qt.