[PATCH] 128-bit ABI for x86_64-w64-mingw32 incorrectly generated

[Jameson Nash]

@rnk I have updated the diff based on your comments (although I did not manage to locate a copy of clang-format). As suggested, I strengthened all of the conditional checks to assertions. The original code to retest the conditional was copied from the ARM code example and was not needed here.

http://reviews.llvm.org/D1998