In the GARBAGE-COLLECT primitive, return the available heap space

In the GARBAGE-COLLECT primitive, return the available heap space
minus gc_space_needed, in case it was called by a primitive that
requested a certain amount of space for allocation.  If we report only
the newly available heap space and ignore gc_space_needed, then
GC-FINISH will consider there to be ample space to proceed (more than
4096 bytes), and the primitive will be restarted only to repeat the
call to the GC, looping endlessly.