As for WinNT and other non-ELF platforms systems, that's retro hobbyist territory, not very likely to end up in upstream LLVM. But you could always convert object files into the right format externally! e.g. for years building TianoCore EDK2 on non-Windows has involved conversions from ELF to PE (now there's also a CLANGPDB setting that produces PEs directly from LLVM)
As for WinNT and other non-ELF platforms systems, that's retro hobbyist territory, not very likely to end up in upstream LLVM. But you could always convert object files into the right format externally! e.g. for years building TianoCore EDK2 on non-Windows has involved conversions from ELF to PE (now there's also a CLANGPDB setting that produces PEs directly from LLVM)