round is sometimes wrong for numbers bigger than most-positive-fixnum

[rtoy]

(round (+ 536870911 1.5d0)) should return 536870912 and 0.5d0. However, cmucl returns 536870913 and -0.5d0.

The code in float.lisp for %unary-round appears to be wrong for numbers outside fixnum range. At the very least we should extend the range to (signed-byte 32), but there would still be issues for other numbers.

[rtoy]

Fixed. Was not checking for odd-valued results that needed to be rounded to even.