score: Workaround for GCC 4.9 for atomic ops

The GCC 4.9 ships its own <stdatomic.h> which is not C++ compatible.
The suggested solution was to include <atomic> in case C++ is used.
This works at least with GCC 4.9.  See also:

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=60932
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=60940
rtems: Delete unused <rtems/rtems/atomic.h>