[libcxxabi] Support building hermetic static library

This is useful when the static libc++abi library is being linked into
shared libraries that may be used in with other shared libraries that
use different C++ library. We want to avoid avoid exporting libc++abi
or libc++ symbols in those cases. This achieved by a new CMake option
which can be enabled by libc++abi vendors as needed.

Differential Revision: https://reviews.llvm.org/D56026

llvm-svn: 352017