Fix exp in non-default rounding modes (bug 3976)

[David Miller]

From: "Joseph S. Myers" <joseph@codesourcery.com>
Date: Wed, 29 Feb 2012 01:51:31 +0000 (UTC)

> No, it's not undefined;

I'm just saying that this is the exact language I've seen in Sun's
documentation for libm, it's "undefined".