Fix parallel build issues and add MK_ZFS=no case

To ensure that I don't break the MK_ZFS=no case (currently working),
add that to the build, at least for i386 and amd64. Also, out of an
abundance of caution, clean single threaded, and then build in a
separate make. Otherwise, I hit a race where we build before objdir
has been created, which breaks at the moment.

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