[llvm] [runtimes] Append `-nostd*++` flags only for Clang (PR #151930)

[Michael Kruse via llvm-commits]

Meinersbur wrote:

> The runtimes order is deterministic so we just move those libraries earlier in the chain. Having the builtins separate only makes sense because oftentimes those are necessary for passing some feature checks by the compiler.

And CMAKE_REQUIRED_FLAGS, that this PR is about, is used for feature checks. When building `runtimes-configure`, the order in the chain is irrelevant, since libc++.so has not been built in any case.

Consider my previous statement:

>  Or, at least this seems to be the current approach, make the configure step not depend on the existence of the C++ standard library

https://github.com/llvm/llvm-project/pull/151930