Support unreleased LLVM 3.3 using svn trunk

LLVM 3.3 is still not released. So, note that LLVM's API may change further and
break buidling Rubinius with LLVM 3.3.
Merge pull request #2242 from ryoqun/support-llvm-3.3

Support unreleased LLVM 3.3 using svn trunk