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

[Andrew Haberlandt via llvm-commits]

ndrewh wrote:

@petrhosek Could you check if you think the approach here is reasonable? And if you have any suggestions for what to do about the builtins tests that are failing (disable them? or any suggestions for who can take a look at fixing them?)

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