[arm] fix exit syscall to match gem5 behavior, vvadd works!
[arm] fix exit syscall to match gem5 behavior, vvadd works!

moreati/pydgin,cornell-brg/pydgin,futurecore/pydgin,moreati/pydgin,futurecore/pydgin,futurecore/pydgin,cornell-brg/pydgin,moreati/pydgin,cornell-brg/pydgin,moreati/pydgin,futurecore/pydgin,cornell-brg/pydgin