Give Holmakefiles access to a RELOCBUILD variable

Use this to avoid give set_mtime executable an adjusted mtime when it
is built in a RELOCBUILD situation.

Let Holmakefiles pass on a --dbg flag if it was passed to Holmake

The standard variable environment that is consulted and updated in a
Holmakefile now starts with

DEBUG_FLAG |-> --dbg

if --dbg was passed to Holmake.

First hack at providing mlton Holmake

Replace 'print' with Holmake's output functions

Make KERNELID var get its proper value in Holmake

Issue was that Poly/ML evaluates many things at compile time rather than
run-time.

Remove diffs between poly and mosml Holmake impls

Next step is to actually remove as much as possible of the
tools-poly/Holmake directory.