testsuite: Fix run to main issue introduced by GCC 5.x.

Adding a dummy assignment as a new breakpoint anchor because
breakpoint on return statement doesn't work for GCC 5.x.

2016-03-02  Bernhard Heckel  <bernhard.heckel@intel.com>

gdb/testsuite/Changelog:

     * gdb.cp/vla-cxx.cc: Insert dummy assignment as anchor for an breakpoint.