[libunwind][CMake] Provide option to disable instalation of the library

This is useful in cases where we only build static library and
libunwind.a is combined with libc++abi.a into a single archive in which
case we don't want to have libunwind.a installed separately. The same
option is already provided by libcxx CMake build.

This change also adds the install-unwind target for consistency with the
libcxxabi and libcxx CMake build.

Differential Revision: https://reviews.llvm.org/D40195

llvm-svn: 318569