localeinit.c: fallback filesystem encoding to UTF-8

* localeinit.c (Init_enc_set_filesystem_encoding): fallback to
  UTF-8 on Mac OS X, just for convenience sake.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@46202 b2dd03c8-39d4-4d8f-98ff-823fe69b080e