Windows builds

[fcremo]

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This PR adds/fixes Windows builds for:

• llvm
• mingw
• gcc

It also switches the libcs from having a special @headers build to depending on a separate component for the headers (this was required by mingw which had to be installed in three phases).

Note: requires features from orchestra PR https://github.com/revng/revng-orchestra/pull/25

[aleclearmind]

Update for when you resume working on this: I've pushed on this repository feature/windows. Please resume from there.

Notes:

• I've created feature/windows branches for ui-qt-creator, llvm, revng and qemu. Some of them have dirty commits, but they should work.

Things that need to be done:

• Factor out x86_64-w64-mingw32-clang into something similar to data.values.regular_c_compiler
• Unify and factor out compiler flags specific for windows. Currently they are in multiple (namely, llvm, boost, ui/qt, ui/qt-creator, cmake.lib.yml) places and are slightly different.
• Important: I have not fixed dependencies, they need to be fixed (e.g., windows builds need to depend on the Windows compiler).
• Factor out $ORCHESTRA_ROOT/x86_64-w64-mingw32.

Some of this tasks call for a more in depth handling of cross-compiling components, but I'd like to postpone that.

[aleclearmind]

We need to integrate the precious suggestions by mstorsjo.