Scan all input files for symbol reference warning

This patch scans all input files for symbol reference warning if the
symbol reference doesn't exist in the current input file.

ld/

	PR ld/16756
	* ldmain.c (symbol_warning): New function.
	(warning_callback): Use it.  Scan all input files for a reference
	to SYMBOL.

ld/testsuite/

	PR ld/16756
	* ld-plugin/lto.exp: Expect filename and line number for PR
	ld/12760 test.