Fix several bugs related to varying the size of constant space:

Fix several bugs related to varying the size of constant space:
- The gc file should be large enough to accomodate all the heap and
  constant space, not only the space indicated by the -heap parameter.

- The size of the gc file should be a multiple of the gc buffer size.
  The garbage collector always writes a multiple of this quantity,
  even though at the end, only the area in use is read.