[MLton] commit r4134: fixed fix to getrusage bug

[Stephen Weeks]

>     val MLton.GC.setRusage: bool -> unit
...
> However, this function was never used in the basis library.  In
> particular, we never enabled rusage when MLton.Rusage.rusage was used.
> This further meant that Timer.getGCTime would _always_ return zero,
> unless the user happened to run with gc-summary or gc-messages (or
> used the corresponding MLton.GC functions).

The fix to GC_setRusage looks correct.  But the fix to
MLton.Rusage.rusage looks incorrect.  It was intentional to not call
"MLtonGC.setRusage true" there.  The problem is that this includes all
the getrusage calls (with noticeable performance cost) just because
rusage occurs in the program.  The intent is to have the programmer
specify at run time whether or not he wants the information gathered.
For example, we do this in MLton with a line in main.fun:

   val () = MLton.GC.setRusage (!verbosity <> Silent)