atomic.h: prefer GCC atomic builtins
git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@36536 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
atomic.h: prefer GCC atomic builtins
since they are generic.
git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@36536 b2dd03c8-39d4-4d8f-98ff-823fe69b080e