In order to be able to build these various targets, we need host tools
to be available to the build, and so must first build _worldtmp to
ensure that host-symlinks is run on a skeleton worldtmp. A few tools we
need are also expected to have (maybe) been built from source, not use
the host version unconditionally (e.g. awk, sed and cat), so we also
need to run _legacy and _bootstrap-tools.
Also, whilst we could previously get away with MAKEOBJDIRPREFIX not
being set for distrib-dirs, we need it with BUILD_WITH_STRICT_TMPPATH as
it determines where worldtmp is, and thus what PATH is set to. Since
every make invocation now needs it, we can just export it up-front
rather than setting it every time (and even if it wasn't needed, it does
no harm and is simplifying).