on x86 - the kernel doesn't really care (as long as it's all flat 32-bit),
and it means that the return path for sysenter/sysexit can avoid re-loading
the segment registers.

NOTE! This means that _all_ kernel code (not just the sysenter path) must
be appropriately changed, since the kernel knows the conventions and doesn't
save/restore DS/ES internally on context switches etc.