Implement portable abs for complex input

Absolute value of a complex number is straightforward enough. Had to fix a couple other things because this is (I think) the first use of complex types in ExecuTorch.

Differential Revision: [D69058051](https://our.internmc.facebook.com/intern/diff/D69058051/)

ghstack-source-id: 264405527
Pull Request resolved: https://github.com/pytorch/executorch/pull/8146

Co-authored-by: Scott Wolchok <swolchok@meta.com>