make struct Event layout consistent on 32/64-bit

To make struct Event have consistent memory layout between x86
and x86_64, force nsecs_t timestamp align to 64bit because it's
not true for x86.

Change-Id: I49eaddd89e10d595cf6a5130f9762b5388c96625
Signed-off-by: Fengwei Yin <fengwei.yin@intel.com>