[cmake] Add MINGW_LIBRARIES to the linker flags

[cmake] Add MINGW_LIBRARIES to the linker flags

This is essential when building with -nodefaultlibs.

In some CMake versions (noticed in 3.5.1), the same libraries are
picked up from CMAKE_REQUIRED_LIBRARIES in some exceptional situations
(if CXX probing failed, due to libc++ not being built yet, the libraries
from CMAKE_REQUIRED_LIBRARIES are used for linking the target library),
but not at all in other newer CMake versions (3.10).

This is similar to what already is done in libcxxabi in SVN r302760
and libcxx in SVN r312498.

Differential Revision: https://reviews.llvm.org/D50663

llvm-svn: 339642