Keep some relocations with undefined weak symbols.

This fixes pr34301.

As the bug points out, we want to keep some relocations with undefined
weak symbols. This means that we cannot always claim that these
symbols are not preemptible as we do now.

Unfortunately, we cannot also just always claim that they are
preemptible. Doing so would, for example, cause us to try to create a
plt entry when we don't even have a dynamic symbol table.

What almost works is to say that weak undefined symbols are
preemptible if and only if we have a dynamic symbol table. Almost
because we don't want to fail the build trying to create a copy
relocation to a weak undefined.

llvm-svn: 313372