alpha: switch to copy_thread_tls()

Use the copy_thread_tls() calling convention which passes tls through a
register. This is required so we can remove the copy_thread{_tls}() split
and remove the HAVE_COPY_THREAD_TLS macro.

Cc: linux-alpha@vger.kernel.org
Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>