* internal.h (bit_length): Add casts to fix complation error with
  clang 3.0 -Werror,-Wshorten-64-to-32.
  [ruby-dev:47687] reported by SASADA Koichi.



git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@42784 b2dd03c8-39d4-4d8f-98ff-823fe69b080e