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/CMakeLists.txt

@@ -67,6 +67,7 @@ set(LIBC_SOURCES
     termios.cpp
     time.cpp
     times.cpp
+    tls.cpp
     ulimit.cpp
     unistd.cpp
     utime.cpp
@@ -99,7 +100,7 @@ file(GLOB ELF_SOURCES CONFIGURE_DEPENDS "../LibELF/*.cpp")
 
 if ("${SERENITY_ARCH}" STREQUAL "aarch64")
     set(ASM_SOURCES "arch/aarch64/setjmp.S")
-    set(ELF_SOURCES ${ELF_SOURCES} ../LibELF/Arch/aarch64/entry.S ../LibELF/Arch/aarch64/plt_trampoline.S)
+    set(ELF_SOURCES ${ELF_SOURCES} ../LibELF/Arch/aarch64/entry.S ../LibELF/Arch/aarch64/plt_trampoline.S ../LibELF/Arch/aarch64/tls.S)
     set(CRTI_SOURCE "arch/aarch64/crti.S")
     set(CRTN_SOURCE "arch/aarch64/crtn.S")
 elseif ("${SERENITY_ARCH}" STREQUAL "x86_64")