This ldexp stub will shadow the ldexp weak alias for scalbn in libm,
which is unfortunate but hard to fix properly without chasing the
mythical libc bump beast. With the change here, we should raise all
the same exceptions that libm's scalbn does -- overflow, underflow,
inexact-result, and (for signalling NaN only) invalid-operation.
This in turn should correct the missing overflow/underflow exceptions
of our portable C fma, and perhaps other routines.
XXX pullup
59ccc04e14