* enc/{ascii,us_ascii,utf_8}.c: set encoding indexes of
  fundamental built-in encodings so that usable as well as
  allocated rb_encoding before rb_enc_init().

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@51862 b2dd03c8-39d4-4d8f-98ff-823fe69b080e