folly | Bypass checking for system preadv and pwritev on iOS Simulator platform
folly | Bypass checking for system preadv and pwritev on iOS Simulator platform

Summary: Fix building folly sysuio portability when targeting the iOS simulator, because the current Xcode 12 SDK does not contain `preadv` and `pwritev` for the iOS simulator on Catalina, even though it declares it's there, so we get link errors.

Reviewed By: yfeldblum

Differential Revision: D22569172

fbshipit-source-id: 870b47063cb37a24364d1de292aba1f966edfb7c

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