Realm-cocoa: Support Project Catalyst/UIKitForMac

[rjchatfield]

Goals

Compile and use Realm for iOS in a Project Catalyst (iPad app for macOS) project.

Expected Results

Currently it compiles, but it just hung when we tried. It should work as it always does ;)

[rursache]

how did you got it running? i keep getting

building for UIKitForMac, but linking in object file built for iOS Simulator, for architecture x86_64

clang: error: linker command failed with exit code 1 (use -v to see invocation)

[jlubeck]

Probably with this: https://github.com/realm/realm-cocoa/pull/6164

I'm getting the same errors as @iPhoNewsRO but I'll see if I can try that pull request

[rjchatfield]

This is a dupe of the #6163 (which seems like a huge ticket!)

[realm-probot[bot]]

Hey - looks like you forgot to add a T:* label - could you please add one?