merge revision(s) 43996:

	* ruby_atomic.h (ATOMIC_PTR_EXCHANGE): atomic exchange function for
	  a generic pointer.


git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/branches/ruby_2_0_0@44317 b2dd03c8-39d4-4d8f-98ff-823fe69b080e