[CI] Use LLVM_ENABLE_RUNTIMES for runtimes builds on Linux
This patch switches us to using LLVM_ENABLE_RUNTIMES rather than using
separate runtimes builds for some reductions in CMake configuration time
and some simplification of the monolithic-linux.sh script.
Reviewers: DavidSpickett, cmtice, lnihlen, Endilll, tstellar
Reviewed By: Endilll, DavidSpickett
Pull Request: https://github.com/llvm/llvm-project/pull/142694
