https://bugs.winehq.org/show_bug.cgi?id=38886
--- Comment #5 from Martin Storsjö martin@martin.st --- This should be fixed in 8fb8cc03c3edb599dd98f369e14a08f899cbff95 (pushed on May 11th) now, requiring Clang 5.0 or newer, which supports __builtin_ms_va_list for aarch64.