Find lto plugin when using gcc-5.0

I originally had this as --print-prog-name and changed back to
--print-file-name to suit older gcc, neglecting to check whether
gcc-5.0 --print-file-name finds the lto plugin.  It doesn't.

	* ld-plugin/lto.exp: Use both --print-file-name and --print-prog-name
	when looking for lto plugin.

ld/testsuite/ChangeLog

@@ -1,3 +1,8 @@
+2014-09-22  Alan Modra  <amodra@gmail.com>
+
+	* ld-plugin/lto.exp: Use both --print-file-name and --print-prog-name
+	when looking for lto plugin.
+
 2014-09-16  Terry Guo  <terry.guo@arm.com>
 
 	* ld-arm/attr-merge-arch-2.d: New test case.

ld/testsuite/ld-plugin/lto.exp

@@ -45,7 +45,10 @@ set plugin_names {
 }
 set plug_opt ""
 foreach plug $plugin_names {
-    set plug_so [run_host_cmd $CC "--print-file-name $plug"]
+    set plug_so [run_host_cmd $CC "--print-prog-name $plug"]
+    if { $plug_so eq $plug } then {
+	set plug_so [run_host_cmd $CC "--print-file-name $plug"]
+    }
     if { $plug_so ne $plug } then {
 	set plug_opt "--plugin $plug_so"
 	break