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.
