[compiler-rt] [llvm] [compiler-rt] Add check-builtins target for LLVM_ENABLE_RUNTIMES builds (PR #166837)

[Andrew Haberlandt via llvm-commits]

ndrewh wrote:

OK, I ran the tests from `main` on an old-style build (LLVM_ENABLE_PROJECTS) and the tests that fail under the new config actually pass under the old build... which suggests I'm doing something wrong here.

https://github.com/llvm/llvm-project/pull/166837