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: https://llvm.org/svn/llvm-project/libcxxabi/trunk@270816 91177308-0d34-0410-b5e6-96231b3b80d8
5 files changed