Fix compiler depedencies for Firefox on Windows

projects/firefox/config still lists mingw-w64 as a dependency.

But, IIRC, it was used only for libssp, which we compile with Clang in mingw-w64-clang, too.

Indeed, I cannot find where we extract it in projects/firefox/build.

Also, we should copy clang only on Linux, since both Windows and macOS copy it in mingw-w64-clang and macosx-toolchain, respectively.

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