[llvm] [CI] Switch to a single runtimes build (PR #131913)

[Michael Kruse via llvm-commits]

Meinersbur wrote:

[libc](https://github.com/llvm/llvm-project/blob/main/.github/workflows/libc-fullbuild-tests.yml) and [libcxx](https://github.com/llvm/llvm-project/blob/main/.github/workflows/libcxx-build-and-test.yaml) already have their own pre-merge tests. They use stock compilers (gcc and clang), so the additional value here is only whether Clang is regressing. If worker load is at capacity, I would even argue there is not need to bootstap-build libcxx/libc at all, for the same reason a 2-stage build of Clang is probably not worth it: A littlle more coverage, but whether can build in principle has already been verified. So this PR looks good to me, but I am not the person to decide how Google spends its resources 

Note that this is different for Flang-RT: It is designed to only work with a build of Flang from the same revision, so one cannot use a pre-compiled flang. 

https://github.com/llvm/llvm-project/pull/131913