* vm_core.h: use __attribute__((unused)) in UNINTIALIZED_VAR on clang
  4.0+ instead of just on 4.2. Clang has supported the unused attribute
  since before version 4, so this should be safe.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@43111 b2dd03c8-39d4-4d8f-98ff-823fe69b080e