Make sure MACHINE(_ARCH) are exported for depend-cleanup.sh

We run depend-cleanup.sh twice during the build. The second time is the
normal run, where we run it under WMAKEENV and thus have CROSSENV's
MACHINE(_ARCH)=${TARGET(_ARCH)} in the environment. However, the first
time is for bootstrap-tools, where it's run under BMAKEENV and we don't
have any assignments to MACHINE(_ARCH) in the environment, meaning the
script sees them as unset. In practice this doesn't matter since the
only use doesn't apply to bootstrap-tools, but it could be a future
issue. Thus, explicitly export them for depend-cleanup.sh and have the
script verify they're set.

Reviewed by: imp
Differential Revision: https://reviews.freebsd.org/D40968