Promote Isabelle's Exn.reraise to Portable

This function keeps Poly/ML's location information from the original
exception intact (otherwise, the location information given to it is
that of the new raise). Under Moscow ML, there is no location
information so reraise is just equal to raise.

Promote more Isabelle concurrency code

Portable gets slightly richer with

make_counter : {inc:int,init:int} -> unit -> int
syncref : 'a -> {get:unit->'a, upd : ('a -> 'b * 'a) -> 'b}

Under Moscow ML these map to obvious operations on references. Under
Poly/ML these map to synchronised variables that are operated on
atomically.

More assorted code tidying.

Allow Moscow ML build to happen using (as yet unreleased) v2.10.

Basic strategy is to make adoption of basis 2002 even more
thorough. In particular, the Timer structure is now as per
the 2002 Basis.

It is possible to detect that one is working with Moscow ML 2.10 or
later by checking the Holmakefile variable $(HAVE_BASIS2002), which is
set (to value "1") when in 2.10 or later. It is unset elsewhere. See
an example of its use in help/src-sml/Holmakefile.

Better document our use of the Susp code from Moscow ML.

Unfortunately, a Moscow ML 2.01 bug (now reported) means we end up
having to use a new name (HOLSusp) to get the dependencies working
properly. Without this change, building the standard kernel on Moscow
ML would always recompile Thm.sml, seemingly because it couldn't see
the non-existent Susp.ui in SIGOBJ (this file only appears there in the
Poly/ML build). There may still be a Holmake bug here too.

Moved a bunch of things from tools-poly/poly to portableML/poly.

Still to do: make Poly Arb{int,num} use Poly's built-in arbitrarily
large integer type. Also want to get rid of the last remaining
"redirect" in tools-poly/poly/poly-init2.ML.

Reorder the initial build sequence so implementation specific files come 1st.

This will make fiddling with the Poly/ML build easier.