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: https://llvm.org/svn/llvm-project/libunwind/trunk@314716 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed