ELF: Export preempted symbols even if there is a dynamic list.

Dynamic lists in an executable are additive, not restrictive, so we
must continue to export preempted symbols even with a dynamic list.

This fixes sanitizer interception of libc symbols (and should also fix
symbol preemption by users of sanitizers).

Differential Revision: https://reviews.llvm.org/D38647

llvm-svn: 315114