Android: Ensure RN_FABRIC_ENABLED is defined when building with Fabric

[empyrical]

Summary

This pull request tweaks the build script for jsiruntime to ensure that RN_FABRIC_ENABLED is defined when building for Android with Fabric enabled, and using the JavascriptCore JS engine. Without it, the createWeakObject and lockWeakObject methods in JSCRuntime.cpp will throw an exception.

Changelog

[Internal] [Changed] - Android: Ensure RN_FABRIC_ENABLED is defined when building with Fabric

Test Plan

RNTester with JSC now builds and runs on Android with Fabric enabled.

[analysis-bot]

Platform	Engine	Arch	Size (bytes)	Diff
ios	-	universal	n/a	--

Base commit: 653d854

[analysis-bot]

Platform	Engine	Arch	Size (bytes)	Diff
android	hermes	arm64-v8a	7,211,189	0
android	hermes	armeabi-v7a	6,860,361	0
android	hermes	x86	7,645,825	0
android	hermes	x86_64	7,536,761	0
android	jsc	arm64-v8a	9,370,146	0
android	jsc	armeabi-v7a	9,011,458	0
android	jsc	x86	9,232,878	0
android	jsc	x86_64	9,810,022	0

Base commit: 653d854

[facebook-github-bot]

@fkgozali has imported this pull request. If you are a Facebook employee, you can view this diff on Phabricator.

[react-native-bot]

This pull request was successfully merged by @empyrical in 300df59.

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