As much as possible, we should avoid using `REQUIRES: maccatalyst_support`
since tests restricted this way are not run in PR tests. Many tests that
exercise macCatalyst behaviors can be run in a macOS configuration, without
full macCatalyst standard library support.
Also, adopt `%target-cpu` lit substitution where appropriate to avoid needless
standard library module rebuilds when running tests locally.
ecc1a2de2c