x86_64 already does this and it seems to work fine. Since stack areas
grow downwards and should always have a guard page, overruns shouldn't
be too much of a concern (for that matter, randomization still makes
a difference here also, it seems.)
This allows us to regain another ~256 MB or so of contiguous address
space (areas smaller than that could still be inserted in the 0x6.. to
0x7.. block, but if you had 10MB of free address space below 0x6..,
and 246MB above it, you couldn't allocate a 256MB block.)
Part of #19592.
Augustin Cavalier
8f149016fb