[compiler-rt] [llvm] Reland check-builtins target for LLVM_ENABLE_RUNTIMES (PR #171941)

[Petr Hosek via llvm-commits]

petrhosek wrote:

> @petrhosek Have you had a chance to diagnose the armhf failures you were seeing with this change? I had a hypothesis [here](https://github.com/llvm/llvm-project/pull/166837#issuecomment-3645339732) but I don't have a setup for building the armhf configuration so I haven't been able to test anything myself.

You're right, that logic is definitely problematic and should be fixed. The reason we haven't ran into this in the normal build though is because elsewhere we use `set_output_name` to correctly construct the output name. I have updated your PR to use it instead of hardcoding the name.

https://github.com/llvm/llvm-project/pull/171941