Fix pow overflow in non-default rounding modes (bug 16315)

[Joseph S. Myers]

On Thu, 26 Jun 2014, Stefan Liebler wrote:

> Hi Joseph,
> 
> on S390 test-ldouble/test-ildoubl fails after testing in all rounding modes.
> (see out-file; the expm1l failures does not belong to this patch)
> But I haven't debugged it yet.

Well, this patch didn't touch ldbl-128/e_powl.c, so I think this indicates 
pre-existing bugs that the newly enabled tests show up (including cases 
where even the sign of the result is wrong).

-- 
Joseph S. Myers
joseph@codesourcery.com