ruby_atomic.h: ATOMIC_CAS on VC6

* ruby_atomic.h (rb_w32_atomic_cas): call InterlockedCompareExchange
  directly.
* ruby_atomic.h (ATOMIC_CAS): fix missing function call.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@41760 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
* 2013-07-04

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@41761 b2dd03c8-39d4-4d8f-98ff-823fe69b080e