Fix i128 div/mod on mingw64

The Win64 docs are very clear that anything larger than 8 bytes is
passed by reference, and GCC MinGW64 honors that for __modti3 and
friends.

Patch by Jameson Nash!

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@208029 91177308-0d34-0410-b5e6-96231b3b80d8