Tests/LibELF: Test loading libraries with dynamic TLS

The setup is a bit peculiar: both the definition and the use site of
these TLS variables have to be in a shared library, otherwise the linker
might relax the global-dynamic access mode to something that doesn't
require a `__tls_get_addr` call.

Tests/LibELF/TLSUse.cpp

@@ -0,0 +1,48 @@
+/*
+ * Copyright (c) 2023, Daniel Bertalan <dani@danielbertalan.dev>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include <LibTest/Macros.h>
+#include <LibThreading/Thread.h>
+
+// Defined in TLSDef.cpp
+extern __thread int one;
+extern __thread int two;
+extern __thread int three;
+[[gnu::tls_model("initial-exec")]] extern __thread int four;
+extern void check_increment_worked();
+
+static void check_initial()
+{
+    EXPECT_EQ(one, 1);
+    EXPECT_EQ(two, 2);
+    EXPECT_EQ(three, 3);
+    EXPECT_EQ(four, 4);
+}
+
+// This checks the basic functionality of thread-local variables:
+// - TLS variables with a static initializer have the correct value on program startup
+// - TLS variables are set to their initial values in a new thread
+// - relocations refer to the correct variables
+// - accessing an initial-exec variable from a DSO works even if
+//   it's not declared as initial-exec at the use site
+// FIXME: Test C++11 thread_local variables with dynamic initializers
+void run_test();
+void run_test()
+{
+    check_initial();
+    ++one;
+    ++two;
+    ++three;
+    ++four;
+    check_increment_worked();
+
+    auto second_thread = Threading::Thread::construct([] {
+        check_initial();
+        return 0;
+    });
+    second_thread->start();
+    (void)second_thread->join();
+}