MinGW-w64 runtime library inclusion (Clang-compiled) for proper Win64 support?

[kink]

> Unfortunately, there are ABI issues, even a minimalistic
>
> __declspec(dllexport) int foo(int a) { return a + 3; }
>
> in the dummy library returns garbage in x64 mode.

Discard that, I was erroneously using a Clang 3.3 build. It works 
using a very recent 3.4 build. A similar dummy function using 
"long double" doesn't work though, but that's not really 
surprising.

Once these ABI issues are fixed, I think the main task would 
consist in emulating all included MinGW C system headers (not 
present or incomplete in MSVC) by custom ones and getting rid of 
superfluous modules - I guess that'd be a manageable effort.