If --dynamic-list is given, only those symbols are preemptible.

This allows combining --dynamic-list and version scripts too. The
version script controls which symbols are visible, and
--dynamic-list controls which of those are preemptible.

Unlike previous versions, undefined symbols are still considered
preemptible, which was the issue breaking the cfi tests.

This fixes pr34053.

llvm-svn: 312806