x86/oprofile: rename kernel parameter for architectural perfmon to arch_perfmon

The short name of the achitecture is 'arch_perfmon'. This patch
changes the kernel parameter to use this name.

Cc: Andi Kleen <andi@firstfloor.org>
Signed-off-by: Robert Richter <robert.richter@amd.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>