32-bit truncation of 64-bit values

[Peter Jeremy]

Having just tracked down an issue caused by a pointer-to-int
truncation, it occurs to me that it might be "instructive" to change
the amd64 memory map so that the botton 4GB of address-space was
unmapped by default (ie code, data, bss, stack and mmap all default to
above 4GB).  I notice that the process space layout has already
changed since 7.x was branched (at least the same executable runs on
7.x and SEGVs on -current due to a truncated pointer) and making
this change might reveal more broken code.

Given how close we are to 8.0, this might need to wait until after
8 is branched.

-- 
Peter Jeremy
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