Add -Wno-global-constructors -Wno-shadow to fft_r2c op target (#8452)

Add -Wno-global-constructors -Wno-shadow to op targets

Need to suppress these warnings to unbreak builds; looks like this is what PyTorch core does.

Differential Revision: [D69470437](https://our.internmc.facebook.com/intern/diff/D69470437/)

ghstack-source-id: 266120755
Pull Request resolved: https://github.com/pytorch/executorch/pull/8427

Co-authored-by: Github Executorch <github_executorch@arm.com>