x86: use default_cpu_mask_to_apicid for 64bit

Impact: cleanup

Use online_mask directly on 64bit too.

Signed-off-by: Yinghai Lu <yinghai@kernel.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: "Eric W. Biederman" <ebiederm@xmission.com>
Cc: Rusty Russell <rusty@rustcorp.com.au>
LKML-Reference: <49C94DAE.9070300@kernel.org>
Signed-off-by: Ingo Molnar <mingo@elte.hu>