Switch to the new MingW ABI.

GCC 4.7 changed the MingW ABI. On the LLVM side it means that sret functions
don't pop the stack.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@197163 91177308-0d34-0410-b5e6-96231b3b80d8