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

Patch by Rafael Espíndola.

This is PR34053.

The implementation is a bit of a hack, given the precise location where
IsPreemtible is set, it cannot be used from
SymbolTable::handleAnonymousVersion.

I could add another method to SymbolTable if you think that would be
better.

Differential Revision: https://reviews.llvm.org/D36499

llvm-svn: 311468