compiler-rt: add support for mingw-w64 in builtins

The is so that we can avoid using libgcc and use compiler-rt with
mingw-w64.

Related driver patch
http://reviews.llvm.org/D11077

I have tested this with mingw-w64 and everything seems to be in order.
I also sent this patch to the mingw-w64 mailing list for them to look at.

Patch by Martell Malone.

Differential Revision: http://reviews.llvm.org/D11085

llvm-svn: 242539