replace x86_read/write_per_cpu with a common function.

x86_read_per_cpu() and its writeish sister are not present in x86_64. So in
this patch, we replace them with __get_cpu_var(), which is present in both

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>