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

[kink]

Okay, I finally got the x87 argument passing to work between 
Clang and LDC after inserting a few lines into Clang's 
WinX86_64ABIInfo::classify method. Calling foo(666) in D returns 
668.5, as expected.
As soon as I've fixed the remaining linking issues for MinGW-w64 
CRT (including most modules and most likely all relevant ones), 
I'll post the required patches for Clang and the CRT so that 
others can experiment too.