PR22751, LTO broken for libgcc libcalls

PR22751, LTO broken for libgcc libcalls

So what was happening was that the file added from libgcc.a during the
rescan was not put on file_chain.  map_input_to_output_sections then
doesn't see the file and its sections are treated as discarded.

The file_chain list pointer bug was caused by that fact that an
archive element claimed by the plugin does not have my_archive set.
Or more correctly, the actual archive element does have my_archive
set, but this bfd is replaced with a dummy bfd that doesn't have
my_archive set.

	PR 22751
	* ldlang.c (find_rescan_insertion): Look past bfds with claim_archive
	set.