Various fixes to how the Boehm GC's interface.

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.
Merge pull request #1533 from rbehrends/fix-boehmgc-interface

Various fixes to how the Boehm GC's interface.