[libcxxabi][CMake] Provide option to disable installing of the library

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

Differential Revision: https://reviews.llvm.org/D40194

llvm-svn: 318568