[llvm] [runtimes] Fix check-runtimes when target-specific compiler-rt tests are disabled (PR #73610)

[Petr Hosek via llvm-commits]

petrhosek wrote:

The alternative approach I had in mind is definining an empty `runtimes-test-depends` target in the runtimes build when `LLVM_INCLUDE_TESTS` is disabled similarly to what we do for the response file: https://github.com/llvm/llvm-project/blob/1459c627f0bc1a4938b5b6a7f4c2bdc1f3ec6a2c/runtimes/CMakeLists.txt#L250-L253
I implemented that approach in #73629.

https://github.com/llvm/llvm-project/pull/73610