FEX page size requirement
## Problem

I am trying to port FEX in a alpha like architecture, which is 8k pagesize only.

It seems that FEX requires 4k pagesize. I implement MemAllocator8k and MemAllocator8k32Bit to handle guest memory syscall.

Currently, the biggest question for me is that what compements of FEXt self , except the guest syscall handler, depends on 4k pagesize.