[runtime] Fix warnings.
[runtime] Remove NO_FINALIZATION.

Replace with check for Boehm where appropriate. In this runtime,
we don't yet support finalization when using Boehm.
[runtime] Defining GC_LINUX_THREADS is redundant.

Via configure, we already pass in -DGC_THREADS which will do
the right thing (and works for all platforms).

We also don't need a Boehm define when using MPS.
[runtime] Add missing platforms.
[runtime] Support building multiple runtimes.

The Makefile now supports building multiple runtimes
for each platform. The runtimes other than HARP are
not yet implemented, so we only build HARP on the
platforms where we use this directory.
[runtime] Remove obsolete function.

dylan_mm_deregister_thread was replaced a long time ago.
[runtime] Split boehm and MPS code up.
[runtime] Separate boehm/mps collector includes.

Building collector.c with Boehm no longer needs MPS includes. This
isn't all as clean as it might be, but we'll get there. There are
also other dependencies on MPS in other files still.
[runtime][boehm] Use macros for GC_MALLOC/FREE.