Find lto plugin when using gcc-5.0

[Alan Modra]

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.

diff --git a/ld/testsuite/ld-plugin/lto.exp b/ld/testsuite/ld-plugin/lto.exp
index 3a4f661..031752d 100644
--- a/ld/testsuite/ld-plugin/lto.exp
+++ b/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

-- 
Alan Modra
Australia Development Lab, IBM