Add --target option to select code generation target
Clang is able to generate code for both MSVC and mingw on Windows:
clang --help
…
--target=<value> Generate code for the given target
Use x86_64-w64-windows-gnu
for mingw compatible object files, x86_64-pc-windows-msvc
for Visual Studio compatible object files, etc.
We should follow the same design.