Turn off -Werror for gcc 4.2.1 for userland

As discussed on arch@, gcc 4.2.1 is on its way out. Turn off Werror on gcc
versions < 5.0 permantly. This will allow older platforms to continue to compile
w/o new errors once we take them out of universe by default. This will also free
developers from chasing down obsolete warnings that produce no beneficial
changes to the source.

Discussed on: arch@
Reviewed by: jhb@, emaste@, pfg@
Differential Revision: https://reviews.freebsd.org/D21378