Implement native TLS support on Windows

GCC 16 introduced native TLS variables on Windows, so this adds
debugger support for them.

The fetch_tls_load_module_address gdbarch method is used to get the
address of _tls_index of the OBJFILE, which is then forwarded as LM_ADDR
to windows_get_thread_local_address.

The TLS slot for a module can be found in
TIB->thread_local_storage[_tls_index].

Approved-By: Tom Tromey <tom@tromey.com>