This mirrors _cleanobj vs _obj, and will be used by etcupdate to ensure
its build tree has host tools available when BUILD_WITH_STRICT_TMPPATH
is enabled by default (whether in the source tree, like CheriBSD, or in
a system config file).
Note that due to line length the .MAKE .PHONY dependency line is split
into two, one for the filtered WMAKE_TGTS and one for the filtered
.ALLTARGETS.