stub: Fix booting with old kernels

stub: Fix booting with old kernels

This fixes a regression introduced in e1636807 that removed setting this
value as it seemingly was not used by the kernel and would actively
break above 4G boots. But old kernels (4.18 in particular) will not boot
properly if it is not filled out by us.
The original issue was using the truncated value to then jump into the
kernel entry point, which we do not do anymore. So setting this value
again on newer kernels is fine.