No need to check version if symbol is unreferenced and undefined.

bfd/

	PR ld/20306
	* elflink.c (elf_link_check_versioned_symbol): Return false
	for unreferenced undefined symbol.

ld/testsuite/

	* testsuite/ld-gc/gc.exp: Run pr20306 test.
	* ld-gc/pr20306.c: New file.
	* ld-gc/pr20306.d: Likewise.