x86: use descriptor's functions instead of inline assembly

This patch provides a new set of functions for managing the descriptor
tables that can be used instead of putting the raw assembly in .c files.

Remodeling of store_tr() suggested by Frederik Deweerdt.

[ tglx: arch/x86 adaptation ]

Signed-off-by: Glauber de Oliveira Costa <gcosta@redhat.com>
Signed-off-by: Andi Kleen <ak@suse.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>