Properly mark the plugin symbol undefined

Properly mark the plugin symbol undefined

Mark the unused plugin defined symbol in elf_link_input_bfd instead of
_bfd_elf_fix_symbol_flags.  Limit the PR ld/12365 test to x86 targets.

bfd/

	PR ld/12365
	PR ld/14272
	* elflink.c (_bfd_elf_fix_symbol_flags): Revert the last change.
	(elf_link_input_bfd): Mark the plugin symbol undefined if it is
	referenced from a non-IR file.

ld/testsuite/

	PR ld/12365
	PR ld/14272
	* ld-plugin/lto.exp: Run the PR ld/12365 test only for x86 targets.
	* ld-plugin/plugin-7.d: Updated.
	* ld-plugin/plugin-8.d: Likewise.