[MLton] latest MLton segfault in gmp

[Florian Weimer]

* Wesley W. Terpstra:

> Sure, that's clear. gcc is welcome to emit aligned output. The question is
> if it is justified to assume that all other modules likewise align their
> output. That's what it did and that's why we crash.

The AMD64 psABI supplement requires 8-byte alignment.  And the end of
the input area on the stack must be aligned to a 16 byte boundary upon
function entry. See <http://www.x86-64.org/documentation/abi-0.99.pdf>.