x86: use the same data type for tls_array.

This patch changes the type of tls_array in x86_64 to
a desc_struct. Now, both i386 and x86_64 tls_array have
the same type, and code accessing it can be shared.

Signed-off-by: Glauber de Oliveira Costa <gcosta@redhat.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>