[CMake][libcxxabi] Support merging objects when statically linking unwinder

When using LLVM unwinder and static unwinder option is set, merge
libunwind and libc++abi objects into a single archive. libc++ already
supports merging libc++abi.a and libc++.a into a single archive; with
this change, it is possible to also include libunwind.a in the same
archive which is useful when doing static link and using libc++ as
a default C++ library and compiler-rt as a default runtime library.

Differential Revision: https://reviews.llvm.org/D39949

llvm-svn: 318563