LibC+LibELF: Support loading shared libraries compiled with dynamic TLS

This is a prerequisite for upstreaming our LLVM patches, as our current
hack forcing `-ftls-model=initial-exec` in the Clang driver is not
acceptable upstream.

Currently, our kernel-managed TLS implementation limits us to only
having a single block of storage for all thread-local variables that's
initialized at load time. This PR merely implements the dynamic TLS
interface (`__tls_get_addr` and TLSDESC) on top of our static TLS
infrastructure. The current model's limitations still stand:
- a single static TLS block is reserved at load time, `dlopen()`-ing
  shared libraries that define thread-local variables might cause us to
  run out of space.
- the initial TLS image is not changeable post-load, so `dlopen()`-ing
  libraries with non-zero-initialized TLS variables is not supported.

The way we repurpose `ti_module` to mean "offset within static TLS
block" instead of "module index" is not ABI-compliant.

Userland/Libraries/LibC/tls.cpp

@@ -0,0 +1,26 @@
+/*
+ * Copyright (c) 2023, Daniel Bertalan <dani@danielbertalan.dev>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include <AK/Types.h>
+#include <sys/internals.h>
+
+extern "C" {
+
+// This function is called to compute the address of a thread-local variable
+// which might not be stored in the static TLS block (local-dynamic and
+// global-dynamic models). Compilers default to this when creating shared
+// libraries, as they may be loaded after program startup by `dlopen()`.
+//
+// We currently only support a static TLS block, so we take a shortcut in the
+// implementation of this interface: instead of storing the module ID in
+// ti_module, we store the module's TLS block offset. This avoids the need to
+// have a per-thread module ID -> TLS block address. This will have to be
+// changed if we support dynamically allocated TLS blocks.
+void* __tls_get_addr(__tls_index* index)
+{
+    return reinterpret_cast<void*>(reinterpret_cast<FlatPtr>(__builtin_thread_pointer()) + index->ti_module + index->ti_offset);
+}
+}