[MLton] latest MLton segfault in gmp

[Florian Weimer]

* Wesley W. Terpstra:

> Before we change MLton's default alignment, I think there is another
> question that needs to be answered: is gcc's behaviour correct? I assume you
> (Matthew) read the amd64 ABI. What exactly are the rules for alignment of
> pointers/word64s? I assumed that they can be 4-byte aligned on amd64, but
> perhaps being inside an array requires they be 8-byte aligned?

8-byte alignment is preferable for performance reasons (so GCC follows
that), but it's not enforced by the hardware.