https://bugs.winehq.org/show_bug.cgi?id=38380
--- Comment #13 from Josh Pettus jshpettus@gmail.com --- At the time, clang could not compile wine64 due to the lack of builtin_ms_va_list
A few months ago, the patch to implement it finally was added to llvm master branch. I've been compiling 64bit versions of wine for a little bit now.