Sync with upstream

Looking at the [repository for luajit2](https://repo.or.cz/luajit-2.0.git) and specifically

https://repo.or.cz/luajit-2.0.git/history/HEAD:/dynasm/dasm_x86.lua  
https://repo.or.cz/luajit-2.0.git/history/HEAD:/dynasm/dasm_x86.h  

there are some fixes that have yet to be applied to our copy of the code.