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

[kink]

PS: For those who don't know: the MinGW C runtime is just an 
additional layer on top of Microsoft's Visual C Runtime, adding 
missing functionality. What we need would ideally be a clean D 
port of missing and relevant MinGW-runtime parts into 
DRuntime/Phobos, for both DMD and LDC on Win64. Since that's 
quite a task and especially because it is only needed for a 
single, yet the most popular OS, my proposal is to include 
relevant parts of the MinGW runtime layer directly with the Win64 
D compiler, so that we have a solid fallback for the time being.