[llvm] [premerge] Build and check runtimes on macOS premerge (PR #210852)

[Andrew Haberlandt via llvm-commits]

ndrewh wrote:

It looks like we're building some iOS/iOS-sim runtimes. That's probably costing a bit of build time (but not test time since they aren't included in `check-compiler-rt`). We could set `COMPILER_RT_ENABLE_IOS=OFF` to shave off a couple of minutes(?) of build-time. We have coverage of those on https://green.lab.llvm.org and I think it's worth sacrificing iOS to decrease the overall time for the job.

https://github.com/llvm/llvm-project/pull/210852