iOS: building for simulator still builds i386 arch

When we build for an iOS simulator, generally I think we have to be on x86_64 machines now. We do intentionally set ONLY_ACTIVE_ARCH to try and speed up builds from targeting unnecessary architectures, but that value is ignored because we're targeting a "generic" simulator target. So we must use EXCLUDE_ARCHS to avoid targeting i386 and/or arm64.

We already have some logic to exclude arm64 if we have modules/frameworks that don't support arm64-simulator. But ideally we should just always exclude i386/arm64 when building for sim on an x86_64 machine (and exclude i386/x86_64 when eventually building on an arm64 Apple Silicon machine). Otherwise build times are doubled/tripled unnecessarily.