sys_ioperm is only implemented on x86 (i386/x86_64).  Make the
ifdefs in vt_ioctl.c more explicit so the other architectures can
get rid of their stubs in favour of just using sys_ni_syscall in
the syscall table.

Maybe we should remove it altogether, it looks like remnants from
some very early X386 times (ie the Thomas Roell timeframe).