[CI] Migrate runtimes projects in premerge to runtimes build

[boomanaiden154]

We need to migrate the runtimes projects like libc to use the runtimes build in the premerge infra as eventually building runtimes using LLVM_ENABLE_PROJECTS will be deprecated (with that planned after LLVM 21 branches).

[llvmbot]

@llvm/issue-subscribers-infrastructure

Author: Aiden Grossman (boomanaiden154)

We need to migrate the runtimes projects like `libc` to use the runtimes build in the premerge infra as eventually building runtimes using `LLVM_ENABLE_PROJECTS` will be deprecated (with that planned after LLVM 21 branches).