builtins: Allow building windows arm functions for mingw

When building with clang/LLVM in MSVC mode, the msvcrt libraries contain
these functions.

When building in a mingw environment, we need to provide them somehow,
e.g. via compiler-rt.

The aeabi divmod functions work in the same way as the corresponding
__rt_*div* functions for windows, but their parameters are swapped.
The functions for converting float to integer and vice versa are the
same as their aeabi equivalents, only with different function names.

Differential Revision: https://reviews.llvm.org/D26183

llvm-svn: 287465