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

[Petr Hosek via llvm-commits]

petrhosek wrote:

This broke our toolchain builders with the following error:
```
CMake Error at /b/s/w/ir/x/w/llvm-llvm-project/compiler-rt/test/builtins/CMakeLists.txt:108 (file):
  file failed to open for reading (No such file or directory):

    /b/s/w/ir/x/w/llvm_build/runtimes/builtins-armv7-unknown-linux-gnueabihf-bins//clang_rt.builtins-armhf.sources.txt
```
I'm going to investigate the issue but would it be possible to revert the change in the meantime? We could consider reverting just https://github.com/llvm/llvm-project/pull/171741 to effectively disable this feature.

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