Fix #40: Move start of heap space higher
This is a workaround for issue #40. By moving the start of the heap to a higher address, we can still run on older systems (albeit with reduced max heap size), and run on newer systesm where the C code is now mapped at or overlapping the (old) heap start. Arbitrarily choose 0x60000000 as a compromise. This also requires moving the foreign linkage start to a different address because the old address overlaps the new C area. Ideally, we could fix this if we could map the heap wherever the OS wants to put it, but we're not there yet. Use boot-2017-04.lisp to bootstrap this change from the 2017-04 snapshot.