pow(double, int)

[Benjamin Kosnik]

> I resurrected an pow(*, int) code for use in libstdc++, unless someone
> has an even faster one lying around.  As seen from the benchmark it
> improves
> performance, especially for small exponents, but also gives 2x for large
> exponents.

great! Gaby said ok, so I checked it in.

> I was confused by the headers; `shadow/bits/std_cmath.h' is meant
> to become a _replacement_ for `bits/std_cmath.h', right?

Your patch should be fine.  Nathan, care to comment? 

-benjamin