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.