Allow building bin/cat on non-FreeBSD systems

cat -l is needed during the installworld phase and other system's cat
don't support that flag. To avoid portability issues when compiling on
Linux/macOS (such as the the direct access to &fp->_mbstate), we
disable the entire multibyte support when building as a boostrap tool.

Reviewed By: brooks, emaste
Differential Revision: https://reviews.freebsd.org/D13939