[libunwind] Add _LIBUNWIND_DISABLE_VISIBILITY_ANNOTATIONS

It's useful to be able to disable visibility annotations entirely; for
example, if we're building libunwind static to include in another library,
and we don't want any libunwind functions getting exported out of that
library.

https://reviews.llvm.org/D34637
Patch from Thomas Anderson <thomasanderson@chromium.org>!

llvm-svn: 306442