PR24873, gcc -flto objects result in --start-group .. --end-group failure to include...

PR24873, gcc -flto objects result in --start-group .. --end-group failure to include --as-needed libraries

Reloading of archives (and checking --as-needed DSOs again) is
disabled until we hit the plugin insert point.  It's necessary to do
that because in a case like lib1.a lto.o lib2.a where lib1.a and
lib2.a contain duplicate symbols, we want the lto.o recompiled object
to pull in objects from lib2.a as necessary, but not from lib1.a.
Unfortunately this heuristic fails when the insert point is inside a
group, because ld actually loads the symbols from the recompiled
object before running over the contours of the script, thus missing
the fact that new undefs appeared in the group.

	PR 24873
	* ldlang.c (plugin_undefs): New static var.
	(open_input_bfds <lang_group_statement_enum>): Loop on
	plugin_undefs and hitting plugin_insert point.
	(lang_process <lto_plugin_active>): Set plugin_undefs.