Kernel+LibELF: Support initializing values of TLS data

Previously, TLS data was always zero-initialized.

To support initializing the values of TLS data, sys$allocate_tls now
receives a buffer with the desired initial data, and copies it to the
master TLS region of the process.

The DynamicLinker gathers the initial TLS image and passes it to
sys$allocate_tls.

We also now require the size passed to sys$allocate_tls to be
page-aligned, to make things easier. Note that this doesn't waste memory
as the TLS data has to be allocated in separate pages anyway.

Userland/Libraries/LibELF/DynamicLoader.h

@@ -78,6 +78,7 @@ public:
     bool is_dynamic() const { return m_elf_image.is_dynamic(); }
 
     static Optional<DynamicObject::SymbolLookupResult> lookup_symbol(const ELF::DynamicObject::Symbol&);
+    void copy_initial_tls_data_into(ByteBuffer& buffer, size_t total_tls_size) const;
 
 private:
     DynamicLoader(int fd, String filename, void* file_data, size_t file_size);