- bug presented on FreeBSD when `/proc` filesystem is not mounted
- bogus `cc` was used as exename, causing incorrect executable spawn
- llvm::sys::fs::getMainExecutable() has platform-specific code
  to get exename and fallback is to use param argv0
- linux fallback is rare because `/proc` is usually mounted
- *BSD fallback is not rare because `/proc` is often not mounted
- macOS doesn't ever fallback: bug cannot present
- windows doesn't ever fallback: bug cannot present
- other Posix will always present