BFD: Add support for more than one plugin in lib/bfd-plugins

BFD: Add support for more than one plugin in lib/bfd-plugins

ar, nm and ranlib currently lack the ability to handle more than one
plugin in lib/bfd-plugins. This patch reshuffles the logic in plugin.c
to add this functionality. One can now place both llvm and gcc plugins
in this directory and have them loaded automatically.
Mixed gcc/llvm archives are also supported (but not very useful until
ld.bfd and ld.gold also would load multiple plugins and use them to
claim different object files).

	PR 17422
	* plugin.c (try_claim): New function. Moved from
	bfd_plugin_object_p.
	(try_load_plugin): Pass through bfd. Add test.
	(load_plugin): Pass through bfd.
	(bfd_plugin_object_p): Move logic to try_claim.