dirdeps.mk allow for TARGET_SPEC_VARS.host

While *most* projects need only DEP_MACHINE for host, there is always
an exception. So we allow for TARGET_SPEC_VARS.host to be a subset of
TARGET_SPEC_VARS. The default will *just work* for most projects.

We set DEP_TARGET_SPEC_VARS and hence DEP_TARGET_SPEC based on
DEP_MACHINE. Allow for M_dep_qual_fixes.host to be different too and
take care to apply the right set.