x86/process: Unify copy_thread_tls()

While looking at the TSS io bitmap it turned out that any change in that
area would require identical changes to copy_thread_tls(). The 32 and 64
bit variants share sufficient code to consolidate them into a common
function to avoid duplication of upcoming modifications.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by: Andy Lutomirski <luto@kernel.org>