Extended Description

Please backport r308950 ([AArch64] Reserve a 16 byte aligned amount of fixed stack for win64 varargs) to 5.0. This fixes an issue with the new win/arm64 calling convention.

The support for targeting win/arm64 itself is still immature, but the calling convention handling in itself should be mature enough, and can be used by Wine for clang >= 5.0 if this is backported.

This carries no regression risk as it only touches code in new codepaths that aren't used before.