lto: Set plugin_format to bfd_plugin_no only if known_used is set

lto: Set plugin_format to bfd_plugin_no only if known_used is set

commit b21318bd2c29fcca8f99c1de7facdaa5cb2e66e2
Author: Joseph Myers <joseph@codesourcery.com>
Date:   Thu May 11 14:31:09 2023 +0000

    Add LDPT_REGISTER_CLAIM_FILE_HOOK_V2 linker plugin hook [GCC PR109128]

added the LDPT_REGISTER_CLAIM_FILE_HOOK_V2 linker plugin hook for offload
support.  Since the V2 linker plugin hook doesn't claim the offload IR if
known_used is unset, set input plugin_format to bfd_plugin_no only if
known_used is set or the V2 linker plugin hook is unused.

	PR ld/33584
	* plugin.c (plugin_object_p): Set plugin_format to bfd_plugin_no
	only if known_used is set or the V2 linker plugin hook is unused.

Signed-off-by: H.J. Lu <hjl.tools@gmail.com>