restore: Restore FPU state

Since at moment we stick with sigreturn restore
we need to form a proper FPU frame and set a pointer
to it inside sigreturn frame.

For this sake we read the FPU image and here are two
cases are possible

 - no fpu data at all -- nothing to restore, simpliest
   case

 - xsave frame is present but the host cpu supports only
   fxsave instruction: we refuse to continue, since it means
   there are no ymm registers on the machine where we're trying
   to restore

 - fxsave frame is present but the host cpu has xsave feature:
   at moment we refuse to continue, requiring complete match
   between "checkpoint and restore hosts", but in real we could
   extend logic and form complete xsave frame from fxsave and
   continue processing

Signed-off-by: Cyrill Gorcunov <gorcunov@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>