As on x86 compat/native sigframe differ, I need to generalize/modify
sigframe macro helpers having in mind:
- SIGFRAME_OFFSET differ between native/compat tasks, so it takes
  sigframe parameter now, which will be used in following patches
  (also renamed it in RT_SIGFRAME_OFFSET to complement other macros)
- RT_SIGFRAME_FPU is now pointer, because each caller takes result's
  address with &RT_SIGFRAME_FPU(...)
- sigreturn_prep_fpu_frame now takes rt_sigframe parameter, as
  address of fpu_state pointer on x86 will depend on native/compat
  frame type, so I check local sigframe's type and count address
  for rsigframe. (See in the very next commit).

Cc: Laurent Dufour <ldufour@linux.vnet.ibm.com>
Cc: Christopher Covington <cov@codeaurora.org>
Cc: Cyrill Gorcunov <gorcunov@openvz.org>
Signed-off-by: Dmitry Safonov <dsafonov@virtuozzo.com>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>