x86: make atomic64_t work like atomic_t

The volatile keyword has already been removed from the declaration of atomic_t
on x86_64.  For consistency, remove it from atomic64_t as well.

[ tglx: arch/x86 adaptation ]

Signed-off-by: Chris Snook <csnook@redhat.com>
Signed-off-by: Andi Kleen <ak@suse.de>
CC: Andi Kleen <andi@firstfloor.org>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>