Put the RSE backing store at a fixed address. This change is triggered

Put the RSE backing store at a fixed address. This change is triggered
by libguile that needs to know the base of the RSE backing store. We
currently do not export the fixed address to userland by means of a
sysctl so user code needs to hardcode it for now. This will be revisited
later.

The RSE backing store is now at the bottom of region 4. The memory stack
is at the top of region 4. This means that the whole region is usable
for the stacks, giving a 61-bit stack space.

Port: lang/guile (depended of x11/gnome2)