[llvm-branch-commits] [CI] Migrate to runtimes build (PR #142696)

[Aiden Grossman via llvm-branch-commits]

https://github.com/boomanaiden154 created https://github.com/llvm/llvm-project/pull/142696

This patch migrates the premerge pipeline to use LLVM_ENABLE_RUNTIMES to
build libc and compiler-rt.