[Flang] Use `signalPassFailure` for unsupported targets

[JohnTitor]

Summary

A fatal error is inconvenience for users, signalPassFailure should be enough.

Fixes #55015

[awarzynski]

Thanks for working on this! Could you investigate the pre-merge CI failures and also include some indication of the behavior "before" and "after" this change (from the user perspective). Thanks!

flang/lib/Optimizer/CodeGen/Target.cpp

case llvm::Triple::ArchType::ppc64le:
     default:
       break;
     case llvm::Triple::OSType::Linux:
       return std::make_unique<TargetPPC64le>(ctx, std::move(trp),
                                              std::move(kindMap));
     }
     break;
   }
-  llvm::report_fatal_error("target not implemented");
+  mlir::emitError(mlir::UnknownLoc::get(&context), "target is not supported");
+  signalPassFailure();
 }