Mark the plugin symbol undefined

Mark the plugin symbol undefined

LTO may optimize out a plugin symbol, which is also referenced by a
non-IR file.  When that happens, we should mark the plugin symbol
undefined.  It isn't the problem since LTO already determined the
symbols in the non-IR file aren't used.

bfd/

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

ld/testsuite/

	PR ld/12365
	* ld-plugin/pr12365a.c: New file.
	* ld-plugin/pr12365b.c: Likewise.
	* ld-plugin/pr12365c.c: Likewise.

	* ld-plugin/lto.exp (lto_link_tests): Prepare for the PR ld/12365
	test.
	Run the PR ld/12365 test.