encdb.c, utf_16_32.h: Unicode with BOM

* enc/encdb.c, enc/utf_16_32.h (ENC_DUMMY_UNICODE): Unicode with BOM
  must be based on big endian variants, so that actual encodings would
  work.  [ruby-core:57318] [Bug #8940]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@43023 b2dd03c8-39d4-4d8f-98ff-823fe69b080e