Use compiler provided atomic builtins as a 'backend' for
MySQL's atomic primitives. The builtins are available on
a handful of platforms and compilers.


configure.in:
  Check if the compiler provides atomic builtins and
  that __sync_lock_test_and_set stores the correct value.
include/atomic/nolock.h:
  Use the atomic builtins if available.
include/atomic/gcc_builtins.h:
  Implement the atomic ADD, SWAP, CAS, STORE (or operation
  optimized away) and LOAD primitives using atomic builtins
  provided by the compiler.