[libunwind] Create install-unwind-stripped target manually

This supports using a newer libunwind with an older installation of LLVM
(whose cmake modules wouldn't have add_llvm_install_targets).

git-svn-id: https://llvm.org/svn/llvm-project/libunwind/trunk@320163 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed