SI-8102 -0.0.abs must equal 0.0

SI-8102 points out that -0.0.abs returns -0.0 (in error). The same issue
exists for -0.0f. This commit fixes the issue for both by delegating to
math.abs.
Merge pull request #3325 from clhodapp/fix/double.abs

Fix  (postfix abs for -0.0)