[Threading] Fix the Linux build.

A few fixes specifically for the Linux build.

rdar://90776105

unittests/Threading/Fatal.cpp

@@ -0,0 +1,29 @@
+//===---Fatal.cpp - Stub for the threading tests --------------------------===//
+//
+// This source file is part of the Swift.org open source project
+//
+// Copyright (c) 2022 Apple Inc. and the Swift project authors
+// Licensed under Apache License v2.0 with Runtime Library Exception
+//
+// See https://swift.org/LICENSE.txt for license information
+// See https://swift.org/CONTRIBUTORS.txt for the list of Swift project authors
+//
+//===----------------------------------------------------------------------===//
+
+#include "swift/Threading/Errors.h"
+
+#include <cstdarg>
+#include <cstdio>
+#include <cstdlib>
+
+SWIFT_ATTRIBUTE_NORETURN
+SWIFT_FORMAT(1, 2)
+void swift::threading::fatal(const char *msg, ...) {
+  va_list val;
+
+  va_start(val, msg);
+  std::vfprintf(stderr, msg, val);
+  va_end(val);
+
+  std::abort();
+}