Support mingw

[sobczyk]

It would be nice to support mingw for windows native builds, as well as linux cross compiles and
with clang support
I know this would imply some deep changes, but would be beneficial to gn-build users

So it would then support

• mingw on windows
• mingw on linux
• clang + mingw runtime on linux
• clang + mingw runtime on windows
• clang + msvc runtime on linux (nice to have, but not necessary)

[timniederhausen]

I'd certainly be open to supporting that, but implementing it is quite a bit of work, which I simply don't have time for.
However, if you or someone else wants to tackle this, I'll help you as best as possible.