[libc] [llvm] [Flang-RT][libc] LLVM-independent unittests (PR #164794)

[David Spickett via llvm-commits]

DavidSpickett wrote:

Now I realise our build needs a combination of this and the OpenMP fix, but I think the logic here makes sense anyway.

https://github.com/llvm/llvm-project/pull/164794