loader.4th dictthreshold too small

The dictthreshold in stand/forth/loader.4th is too small
resulting in full dictionary.

Reviewed by: stevek, imp
Sponsored by: Juniper Networks, Inc.
Differential Revision: https://reviews.freebsd.org/D44414

(cherry picked from commit a8eb3b365eb63d2a569b166f2dfc982967d3a7fa)