* ruby_atomic.h: use __atomic builtin functions supported by GCC.
  __sync family are legacy functions now and it is recommended
  that new code use the __atomic functions.
  http://gcc.gnu.org/onlinedocs/gcc/_005f_005fatomic-Builtins.html

* configure.in: check existence of __atomic functions.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@43851 b2dd03c8-39d4-4d8f-98ff-823fe69b080e