posix: Avoid NULL pointer access
add atomic init function
smptests: Move ATOMIC conditional to top-level
score: Use static inline directly

This file uses already C11 features.  Include proper header file.
score: Simplify <rtems/score/cpuatomic.h>

Add proper license and copyright.
score: Atomic flag changes

Delete _Atomic_Init_flag().  Change ATOMIC_INITIALIZER_FLAG into a
constant.  Rename _Atomic_Clear_flag() to _Atomic_Flag_clear().  Rename
_Atomic_Test_set_flag() to _Atomic_Flag_test_and_set().  This is now in
line with the C11 schema.
smptests/smpatomic08: New test