Flag on stack frame that segment registers have been saved.

This now completes FreeBSD/Xen pv bringup for UP.

Next steps (open list):
- Optimise current call paths.
  (exception.S, amortise across multiple hypercalls etc.)
- SMP
- vet exception.S (and others) for security.
- Bring codebase as close to native as possible, and merge
  as much code as possible with native and pvhvm.
- Merge to -current

Approved by:	gibbs(implicit)