Fix two issues with the partial gc code.

* Eliminate a fence error in bitmap aliasing.  The heap structure
  uses asymetric bounds but the bitmap structure uses symmetric bounds.
  Bias accordingly.

* Explicitly covert the immune limit to a uintptr_t to muffle a compiler
  warning.

Change-Id: I05e74ab57035ee06eb6d88e773b1d680531a7e2f
2 files changed