[MLton] Darwin/PPC changes

[Daniel C. Wang]

I assume gprof is supported on mach/darwin and is doing PC sampling. 
Perhaps, some digging in the gprof sources will enlighten the issue...

Henry Cejtin wrote:

> As I recall (from long ago playing on Mach) the point about _end and _etext
> and _edata being problematic was that shared libraries got more random
> addresses, but it shouldn't cause a problem for non-shared-library code,
> which is all we get under Linux.  The key point is to see if the difference
> between get_etext() and the lowest use code address (how you get that varies
> with the OS) is close to the code size as printed out by the size command.
> If it is, then using that should be fine.
>