Add lround test for bug 2561

[Joseph S. Myers]

In the absence of any comments on this patch I've now committed it as 
obvious (certainly moving the llround test to the right function is 
obvious).  We can always add a second LONG_MAX conditional in future if 
desired (but I think such conditionals in these tests are generally 
pragmatic - avoiding tests running on supported targets that don't meet 
the conditions of the test in question, not allowing for a hypothetical 
glibc port to an architecture with non-power-of-2-size "long" which is 
the only case where a different LONG_MAX conditional could actually be 
needed for this test).

-- 
Joseph S. Myers
joseph@codesourcery.com