[CMake][libunwind] Define add_target_flags which is missing
[CMake][libunwind] Define add_target_flags which is missing

It's use was introduced in r353084 but its definition is missing.

Differential Revision: https://reviews.llvm.org/D58745

git-svn-id: 62ef012455b6b8fcec9c41ca270ebf409aae1006@355142 91177308-0d34-0410-b5e6-96231b3b80d8

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