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/DynamicLoader/CMakeLists.txt

@@ -13,6 +13,8 @@ file(GLOB LIBC_SOURCES3 "../Libraries/LibC/arch/${ARCH_FOLDER}/*.S")
 set(ELF_SOURCES ${ELF_SOURCES} "../Libraries/LibELF/Arch/${ARCH_FOLDER}/entry.S" "../Libraries/LibELF/Arch/${ARCH_FOLDER}/plt_trampoline.S")
 if ("${SERENITY_ARCH}" STREQUAL "x86_64")
     set(LIBC_SOURCES3 ${LIBC_SOURCES3} "../Libraries/LibC/arch/x86_64/memset.cpp")
+elseif ("${SERENITY_ARCH}" STREQUAL "aarch64")
+    set(ELF_SOURCES ${ELF_SOURCES} "../Libraries/LibELF/Arch/aarch64/tls.S")
 endif()
 
 file(GLOB LIBSYSTEM_SOURCES "../Libraries/LibSystem/*.cpp")