Force RCT_REMOVE_LEGACY_ARCH=1 for the entire OSS iOS framework - WIP !!

[christophpurrer]

Summary:
The legacy (Paper) architecture has been removed from the React Native iOS sources. The remaining call sites are all behind #ifndef RCT_REMOVE_LEGACY_ARCH guards, so leaving the macro unset (or set to 0) now produces an unbuildable framework. Today the CocoaPods plumbing in react_native_pods.rb still treats RCT_REMOVE_LEGACY_ARCH as an opt-out env var ('0' ? '0' : '1'), and Package.swift already hard-codes the macro to 1 for the SwiftPM flow. This diff brings the CocoaPods flow in line with SwiftPM and makes the OSS framework always build with the legacy arch compiled out.

Changes in packages/react-native/scripts/react_native_pods.rb:

• prepare_react_native_project! hard-sets ENV['RCT_REMOVE_LEGACY_ARCH'] = '1' at the top, before any other helper consults the variable.
• use_react_native! re-asserts ENV['RCT_REMOVE_LEGACY_ARCH'] = '1' (the previous ... == '0' ? '0' : '1' opt-out is gone) so consumers that skip prepare_react_native_project! still get the flag.
• react_native_post_install unconditionally adds -DRCT_REMOVE_LEGACY_ARCH=1 to every project's OTHER_CFLAGS / OTHER_CPLUSPLUSFLAGS (the old if/else that could remove the flag is gone).
• New helper set_remove_legacy_arch_compiler_flag!(spec) appends -DRCT_REMOVE_LEGACY_ARCH=1 to a podspec's compiler_flags, preserving anything already there (e.g. js_engine_flags() in React-Core.podspec).

The new helper is invoked once per Meta-owned React-* podspec under packages/react-native/ (72 podspecs in total: every podspec that is not a third-party shim, not a codegen test fixture, not hermes-engine, and not Yoga). Baking the macro into each podspec's compiler_flags mirrors the .define("RCT_REMOVE_LEGACY_ARCH", to: "1") already present in Package.swift and provides a defensive belt-and-suspenders even for consumers that skip react_native_post_install.

Finally, the per-app ENV['RCT_REMOVE_LEGACY_ARCH'] = '1' override that previously sat at the top of packages/rn-tester/Podfile is now redundant and is removed; RNTester picks the flag up from the framework defaults like every other OSS app.

OSS impact: any app consuming react_native_pods.rb will always build React Native with -DRCT_REMOVE_LEGACY_ARCH=1. This matches the iOS source state. The env-var opt-out is gone -- setting RCT_REMOVE_LEGACY_ARCH=0 in the shell or CI no longer has any effect on OSS CocoaPods or SwiftPM builds.

Changelog:
[iOS][Changed] - Force RCT_REMOVE_LEGACY_ARCH=1 for the entire React Native iOS framework in OSS builds; the legacy (Paper) architecture is no longer a supported configuration. The RCT_REMOVE_LEGACY_ARCH=0 env-var opt-out has been removed from react_native_pods.rb.

Differential Revision: D105118393

[meta-codesync]

@christophpurrer has exported this pull request. If you are a Meta employee, you can view the originating Diff in D105118393.