This fixes the pnpbios CS check to check for the correct values (it
wasn't up to date with the various GDT reshuffles), moves it inside the
kernel mode check, modifies it so that it takes less instructions and
marks it with unlikely().

Note that the 2.5.32 version of this check will cause the kernel to
always panic since it checks for the kernel segments and will thus
decide to jump to the pnpbios fault handler without being in pnpbios.
pnpbios_core.c instead seems to use the correct values.