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

[liao jun via llvm-commits]

zjlcd wrote:

Hi @vzakhari, @wangzpgi , @sscalpone and @jeanPerier , I've submitted a PR for standalone llvm flang-rt build。Could you please help review this change when you have a moment?  Thank you!”

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