[Compiler-rt] Downgrade another fatal error to warning

https://reviews.llvm.org/D101681 landed a change to check the testing
configuration which relies on using the `-print-runtime-dir` flag of
clang to determine where the runtime testing library is.

The patch treated not being able to find the path reported by clang
as an error. Unfortunately this seems to break the
`llvm-clang-win-x-aarch64` bot. Either the bot is misconfigured or
clang is reporting a bogus path.

To temporarily unbreak the bot downgrade the fatal error to a warning.
While we're here also print information about the command used to
determine the path to aid debugging.