Add CMake support for building for MinGW
Add CMake support for building for MinGW

This section is similar to what already exists in libcxx and libcxxabi.

Differential Revision: https://reviews.llvm.org/D38380

git-svn-id: 62ef012455b6b8fcec9c41ca270ebf409aae1006@314716 91177308-0d34-0410-b5e6-96231b3b80d8

llvm-mirror/libunwind,llvm-mirror/libunwind,llvm-mirror/libunwind