[MLton] Patch for x86-darwin

[Matthew Fluet]

>> Of course, I don't quite see the point of 16-byte alignment if no function 
>> can assume that the stack pointer is 16-byte aligned on entry.
>
> GCC will always generate stack frames such that they are 16-byte alignment 
> preserving.  This is usually sufficient, as most people don't write assembly 
> code, so every function will be alignment-preserving.  If you disassemble 
> some code, you can actually see that it wastes a little space here and there.
...
> The "right" way would be to write a function entirely in 
> assembly that does the jump, and call it from C, and have it preserve the 
> 16-byte alignment.

Agreed.