Add build support for llvm-mingw toolchain (clang14) on Windows

This MR makes the changes necessary to build Krita and deps with the llvm-mingw toolchain (no MSYS).

Build Instructions

1. Download the llvm-mingw ucrt toolchain with LLVM 13.0.0 14.0.0 and extract it somewhere:
   • Release page: https://github.com/mstorsjo/llvm-mingw/releases/tag/20211002
   • llvm-mingw-20211002-ucrt-x86_64.zip
   • Release page: https://github.com/mstorsjo/llvm-mingw/releases/tag/20220323
   • llvm-mingw-20220323-ucrt-x86_64.zip
2. Set up the build environment like how you would with the mingw-builds toolchain, but add ...\llvm-mingw-20211002-ucrt-x86_64\bin to your PATH instead of mingw-w64 gcc's bin.
3. Call build.cmd with the usual cmdline (build both deps and Krita).
4. (optional) Package with package-complete.cmd.

Running the Build

Note that the runtime DLLs for the built application are placed separately, so if you want to run Krita without packaging you need to also add ...\llvm-mingw-20220323-ucrt-x86_64\x86_64-w64-mingw32\bin to PATH (libwinpthread-1.dll is only available in there).

/cc @dkazakov @lsegovia

Note: Don't merge until 5.0.4 has been released.