* vm_core.h: use __has_attribute() instead of __clang__major__ because
  clang says "Note that marketing version numbers should not be used
  to check for language features, as different vendors use different
  numbering schemes. Instead, use the Feature Checking Macros."
  http://clang.llvm.org/docs/LanguageExtensions.html

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@43116 b2dd03c8-39d4-4d8f-98ff-823fe69b080e