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This commit adds a mingw-w64/clang toolchain that is needed for getting Stylo compiled. The build script follows mostly https://hg.mozilla.org/releases/mozilla-esr68/file/2f459603370d1d35a56ca4e9cee21f7766bbb832/taskcluster/scripts/misc/build-clang-8-mingw.sh Exceptions are: 1) We don't build clang in 3 stages but just one like we do for our macOS cross-compilation toolchain. 2) We are omitting the `DEBUG_FLAGS` (-g -gcodeview) as we plan to actually ship the resulting build to users, which seems okay (see bug 1500102). 3) We use a slightly newer libunwind (8.0.0) to better fit to our clang 8 we deploy. 4) We poke clang harder to compile `i686-w64-mingw32-windres` by specifying the target explicitly. Using our `var/setarch` trick does not work anymore for an unknown reason. Thanks to Martin Storsjö, Jacek Caban, Tom Ritter, and many others for their help in setting this Windows cross-compilation toolchain up.