Fix compilation crash on iOS<16

[f-meloni]

Summary: Added conditional compilation to mitigate a crash on iOS<16 where the used function is not present. Fixes https://github.com/pytorch/executorch/issues/6984

Bonus: report back the error in case of compilation errors, which was missing

Differential Revision: D66294248

[pytorch-bot[bot]]

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:test_tube: See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/executorch/7007

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As of commit 8f75eefe45d735169da98adb2c2e48da1b5d8e76 with merge base 270d0558cecba173f5bc5a667124a79c6258054a (): :green_heart: Looks good so far! There are no failures yet. :green_heart:

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This pull request was exported from Phabricator. Differential Revision: D66294248

[f-meloni]

@pytorchbot label "module: coreml"

[f-meloni]

@pytorchbot label "release notes: backends"