Exclude `i386` from valid architectures when building with Hermes on iOS (#30592)

Summary:
When building React Native application in Release mode for an iPhone Simulator _and_ targeting `armv7`, Xcode will build all architectures (due to `ONLY_ACTIVE_ARCH` set to `false`, unlike in Debug mode). As a result, Xcode will try building for `i386` (32-bit iPhone Simulator), which fails as we don’t build Hermes binaries for `i386`.

Fix is to disable `i386`, since it is not supported by `Hermes` and certain `Folly` features.

## Changelog

[IOS] [BREAKING] - `i386` architecture will be automatically disabled when Hermes is being used. This might be potentially breaking for your workflow if you target `armv7` devices, as you will no longer be able to test on the simulator.
[IOS] [FEATURE] - Replace `flipper_post_install` with `react_native_post_install` hook. Will automatically detect if Flipper is enabled.

Pull Request resolved: https://github.com/facebook/react-native/pull/30592

Test Plan: Run React Native application with Hermes enabled (or Flipper) in Release mode and it should work just fine.

Reviewed By: appden

Differential Revision: D25564738

Pulled By: TheSavior

fbshipit-source-id: e786ab73fb0a77de5869cf9e5999726c7d29f1d4