[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.