fix lightud for 48 bit VA

Hi, all, I've created a pull request in LuaJIT upstream for fixing lightud for 48 bit VA:

https://www.freelists.org/post/luajit/fix-lightud-type-for-48bit-virtual-add...

Please help take a look if you have time, thanks!

Because this issue introduces changes in generic code base and may also affect other platform with 48 bit virtual address configuration, I feel discuss it in upstream may be more appropriate.