x86: change gdt acessor macro name

This patch changes the name of x86_64 macro used to access the per-cpu
gdt. It is now equal to the i386 version, which will allow code to 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>