[compiler-rt] Only query llvm-config in the orc tests

This check for assertions is only used inside the test/orc directory, but
doing it in the top level lit config means all testsuites depend on
llvm-config being present. This is not necessarily needed e.g. when
testing just the builtins. While touching this code, simplify it a bit
by using subprocess.check_output() instead of Popen() and use a string
comparison instead of a regex match.

Reviewed By: lhames

Pull Request: https://github.com/llvm/llvm-project/pull/83705