[PATCH][PPC] Remove unnecessary local variable mptwo

[Roland McGrath]

> > It's an initialized non-static local variable that has to be
> > constructed on every invocation of __inv.  Replacing that with a
> > reference to a hidden global variable has got to be an improvement.
> 
> Right, that was the rationale.  I haven't done an actual comparison of
> the generated code since it's quite straightforward, but I can do that
> tomorrow if necessary.

It seems appropriate to check.  In many cases an initialized local like
that gets folded into its uses and occupies no stack space and uses less
code compared to the load from a global.