x86-64, kcmp: The kcmp system call can be common

We already use the same system call handler for i386 and x86-64, there
is absolutely no reason x32 can't use the same system call, too.

Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>
Cc: H.J. Lu <hjl.tools@gmail.com>
Cc: Cyrill Gorcunov <gorcunov@openvz.org>
Cc: <stable@vger.kernel.org> v3.5
Link: http://lkml.kernel.org/n/tip-vwzk3qbcr3yjyxjg2j38vgy9@git.kernel.org