sim/moxie: add custom directory stamp rule

sim/moxie: add custom directory stamp rule

Because sim/moxie/moxie-gdb.dtb is neither a program nor a library, automake
does not generate dirstamp file ($builddir/sim/moxie/.dirstamp) for it.

When maintainer mode is enabled, it tries to rebuild sim/moxie/moxie-gdb.dtb
but fails because there's no rules for automake-generated dirstamp file
which moxie-gdb.dtb depends.

This commit adds its own rule for the directory stamp (modified copy of the
automake output) and adds the directory stamp file to DISTCLEANFILES to
mimic automake-generated behavior (although "make distclean" does not work
when maintainer mode is enabled).