Makefile.inc1: Only copy locales to INSTALLTMP on FreeBSD

Makefile.inc1 copies locales to ${INSTALLTMP} to avoid issues when
running make installworld on a live system. However, this can break
on non-FreeBSD systems, e.g. on openSUSE where /usr/share/locales
has mode 0555, which means after we copy it, we can't delete it,
so the build fails.

Since this functionality is only useful when installing over a
live system, disable it when the build host is not FreeBSD.

MFC after: 2 weeks
Reviewed by: kevans, emaste
Sponsored by: https://www.patreon.com/bsdivy
Differential Revision: https://reviews.freebsd.org/D56677