Fix some confusion about what can be preempted.

Fix some confusion about what can be preempted.

For shared libraries we allow any weak undefined symbol to eventually be
resolved, even if we never see a definition in another .so. This matches
the behavior when handling other undefined symbols in a shared library.

For executables, we require seeing a definition in a .so or resolve it
to zero. This is also similar to how non weak symbols are handled.

llvm-svn: 262017