[native] Fix iOS build for XCode 12.5

Summary:
Based on:

1. This commit on react-native master
2. This comment
3. The code comment about needing to target iOS 9 in this comment
4. Excluding i386 (32-bit simulators) going forward since they have issues with the threadlocal C++ keyword on iOS 9 release mode builds

Test Plan: In combination with D1088 and D1089, confirm that I can build iOS as (dev, release, archive) on (simulator, device)

Reviewers: palys-swm

Subscribers: KatPo, Adrian, atul

Differential Revision: https://phabricator.ashoat.com/D1087