[llvm] [flang-rt][build] Add support for building flang-rt as a standalone project (PR #168321)

[Michael Kruse via llvm-commits]

Meinersbur wrote:

Building Flang-Rt in a "default build" (this is how the libcxx folks decided to a call a build with https://github.com/llvm/llvm-project/blob/main/runtimes/CMakeLists.txt as top-level build instruction) already works and is documented. What does this PR improve in top of that?

https://github.com/llvm/llvm-project/pull/168321