Revert "x86: Force-disable (f)xsave"
This reverts commit aab0d67ef7.
After investigating, it turns out that the culprit is not xsave, but
xsaves (or rather xrstors64). This is a CPL=0 instruction which
therefore cannot be executed by L4Linux.
Passing the command line option "noxsaves" fixes the issue. I am not
sure whether it makes any sense to even consider using xsaves/xrstors
when L4Linux is running in user space, but maybe there are environments
where it is running with CPL=0 in a VM, so I will leave it as it is.