[PATCH, testsuite]: Fix g++.dg/torture/pr86763.C link failure for glibc < 2.17

[Uros Bizjak]

2018-08-06  Uros Bizjak  <ubiz...@gmail.com>

    * g++.dg/torture/pr86763.C (dg-additional-options): Add -lrt.

Tested on CentOS 5.10 and Fedora 28.

OK for mainline?

Uros.

Index: g++.dg/torture/pr86763.C
===================================================================
--- g++.dg/torture/pr86763.C    (revision 263317)
+++ g++.dg/torture/pr86763.C    (working copy)
@@ -1,5 +1,5 @@
 // { dg-do run }
-// { dg-additional-options "-fschedule-insns2 -fstrict-aliasing" }
+// { dg-additional-options "-fschedule-insns2 -fstrict-aliasing -lrt" }
 
 #include <cstdint>
 #include <cassert>