ruby_atomic.h: suppress warnigns

* ruby_atomic.h (ATOMIC_CAS): suppress C4022 and C4047 warnings in
  VC6.  only InterlockedCompareExchange is declared using PVOID.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@41759 b2dd03c8-39d4-4d8f-98ff-823fe69b080e