I’m not seeing a way of running builtins tests with the runtimes build. This has no check-builtins equivalent of the runtimes version of check-runtimes/check-compiler-rt here
That’s an unintended omission, we should be exporting the check-builtins target. I have filed It's not possible to run builtins tests in the bootstrapping build · Issue #55936 · llvm/llvm-project · GitHub to track this.