[Cocoa] Unify fallback locale string codepath across Cocoa platforms
<rdar://problem/104067282>
Pull request: https://github.com/WebKit/WebKit/pull/8445
Committed 258807@main (c59127dd7d81): <https://commits.webkit.org/258807@main> Reviewed commits have been landed. Closing PR #8445 and removing active labels.