[llvm-branch-commits] [CI] Use LLVM_ENABLE_RUNTIMES for runtimes builds on Linux (PR #142694)

[Aiden Grossman via llvm-branch-commits]

https://github.com/boomanaiden154 created https://github.com/llvm/llvm-project/pull/142694

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.