gcc: Link with the `.lo' file of dependencies, not the `.o' file.

* gcc-plugin/tests/run-test.in (compile/match)[c->o]: Return the `.lo'
  file, not the `.o' file.

gcc-plugin/tests/run-test.in

@@ -283,7 +283,7 @@ otherwise."
 
   (define (c->o c-file)
     (string-append (substring c-file 0 (- (string-length c-file) 2))
-                   ".o"))
+                   ".lo"))
 
   (log "~a directives found in `~a'" (length directives) file)