The Boehm GC interface did not define the getXXXSharedMem() functions
that were needed for compilation with --threads:on. It also used
`ppointer` instead of `PPointer`, so it failed to compile with
--cs:partial.
This revision is intended as comparison point between the old and the new GC
The used GC can be switched in mmdisp and various statistics will be gathered during
execution (these will be removed/disabled in later revisions)