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

[Andrew Haberlandt via llvm-commits]

ndrewh wrote:

```
2025-11-06T20:08:49.4373483Z FAIL: Builtins-x86_64-linux :: extendhfsf2_test.c (6375 of 6511)
2025-11-06T20:08:49.4376218Z ******************** TEST 'Builtins-x86_64-linux :: extendhfsf2_test.c' FAILED ********************
```

Although this could be that I broke the test config somehow, it seems also possible that these tests just weren't being run in CI, and so someone broke them and it was undetected?

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