libc++abi: build with -fvisibility=hidden

Enable building libc++abi with hidden visibility by default.  The ABI mandated
interfaces (and a few extra) are already set up to be externally visible.  This
allows us to ensure that any implementation details are not leaked.

git-svn-id: svn://svn.chromium.org/llvm-project/libcxxabi/trunk@270816 0b72dbe1-c17f-4bc7-b9db-2b4152be0356