[MLton] max-heap setting for 64-bit applications

[Florian Weimer]

* Matthew Fluet:

> (Or, on Windows, use the reserve/commit distinction.

It's there on Linx, too.

> On Posix, use the madvise to preemptively indicate page-out/page-in
> behavior.

Doesn't work.  You have to map with PROT_NONE and then use mprotect to
make available the parts you need.

This is not a published kernel ABI, so it might break in the future.