sim, x86: Make clone a virtual function

sim, x86: Make clone a virtual function

This fixes the function call to clone in syscall_emul.hh where
the x86 version should be called before the base implementation
of clone.

Change-Id: Iccd2f680ff6e3a5536037d688a80ab3f236bbd98
Signed-off-by: Sean Wilson <spwilson2@wisc.edu>
Reviewed-on: https://gem5-review.googlesource.com/3902
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Maintainer: Jason Lowe-Power <jason@lowepower.com>