Create rn_codegen BUCK macro

[RSNara]

Summary: Changelog: [Internal]

Differential Revision: D27669950

[facebook-github-bot]

This pull request was exported from Phabricator. Differential Revision: D27669950

[pull-bot]

	Messages
📖	📋 Missing Changelog - Can you add a Changelog? To do so, add a "## Changelog" section to your PR description. A changelog entry has the following format: [CATEGORY] [TYPE] - Message. CATEGORY may be: General iOS Android JavaScript Internal (for changes that do not need to be called out in the release notes) TYPE may be: Added, for new features. Changed, for changes in existing functionality. Deprecated, for soon-to-be removed features. Removed, for now removed features. Fixed, for any bug fixes. Security, in case of vulnerabilities. MESSAGE may answer "what and why" on a feature level. Use this to briefly tell React Native users about notable changes.
📖	📋 Missing Test Plan - Can you add a Test Plan? To do so, add a "## Test Plan" section to your PR description. A Test Plan lets us know how these changes were tested.
📖	📋 Missing Summary - Can you add a Summary? To do so, add a "## Summary" section to your PR description. This is a good place to explain the motivation for making this change.

Generated by 🚫 dangerJS against a2827e3

[analysis-bot]

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

Base commit: 0cab446

[facebook-github-bot]

This pull request was exported from Phabricator. Differential Revision: D27669950

[analysis-bot]

Platform	Engine	Arch	Size (bytes)	Diff
android	hermes	arm64-v8a	8,852,001	+29,419
android	hermes	armeabi-v7a	8,373,503	+15,765
android	hermes	x86	9,308,702	+29,224
android	hermes	x86_64	9,251,542	+34,869
android	jsc	arm64-v8a	10,582,930	+29,428
android	jsc	armeabi-v7a	10,087,528	+15,763
android	jsc	x86	10,601,126	+29,218
android	jsc	x86_64	11,184,237	+34,863

Base commit: 0cab446

[facebook-github-bot]

This pull request has been merged in e1eb938.