Allow tools/build to work for DIRDEPS_BUILD

We want to be able to build some host tools for non-FreeBSD hosts
without building more than we need. The DIRDEPS_BUILD lets us
do that.

We use the pseudo MACHINE "host" when building
for the host - deal with that when setting _host_arch.

Reviewed by: jrtc27
Sponsored by: Juniper Networks, Inc.
Differential Revision: https://reviews.freebsd.org/D39751