Move CU queue to dwarf2_per_objfile

The CU queue is a member of dwarf2_per_bfd, but it is only used when
expanding CUs.  Also, the dwarf2_per_objfile destructor checks the
queue -- however, if the per-BFD object is destroyed first, this will
not work.  This was pointed out Lancelot as fallout from the patch to
rewrite the registry system.

This patch avoids this problem by moving the queue to the per-objfile
object.