Fix problem where undef can fail to trigger archive rescan.

Fix problem where undef can fail to trigger archive rescan.

If a shared library contains an undefined symbol and LTO adds
a new reference to that same undefined symbol, the reference in the new
object added by the plugin would not trigger a rescan of the archive
containing the symbol.

2017-11-17  Stephen Crane  <sjc@immunant.com>

gold/
	PR gold/22448
	* symtab.cc (Symbol_table::add_from_object): Only rescan for
	undefined symbols in regular, not dynamic, objects.