Fix build of Windows cross-compiler (#20697, #22805)

Fix building of a Windows cross-compiler with Hadrian. First step towards #20697.

• Fixed on Linux (tested on ArchLinux)
• check that it also fixes cross-compilation from Darwin (first commit about AR; needs someone to check that it works): cf #22805 (closed)
• check that third commit makes sense for W32 (whatever that is) and doesn't break CI