Add ECV flag on macOS/iOS

[mscdex]

This flag comes directly from Apple's own page on sysctl names.

[google-cla]

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[mscdex]

Erm, well, it doesn't seem like I'm able to sign the CLA because Google doesn't allow me to select which email address(es) I'd like to use when setting up the agreement -- it's either all or nothing.

[mscdex]

Also, I'm not concerned about attribution or anything so feel free to make the changes manually, it's no big deal. I just want to be able to drop the rest of the patch I currently have on top of this library.