In 6ef6e7c6a74729a6a6bae4f9ba39e216ad13ac70 some support was added for these iOS variants. This commit finishes the job.
After this I'll have a third commit that goes further on making a macos and ios, and both imply darwin. This effectively fixes a regression where before darwin would be used for ios, so darwin_HOST_OS should continue to be true when building for iOS. Since it's also a bug fix, I hope that can also be backported to 8.4.
Also, since that pretty much subsumes this, I'd be happy to just make it part of this rather than do a separate PR. I do think for future reading, having the commits separate is more readable though, and sadly Phabricator squashes everything.