FPU: introduced the support for multiple FPU architectures.

* The routine sigreturn_prep_xsave_frame() is renamed to sigreturn_prep_fpu_frame().

* Moved the routines sigreturn_prep_fpu_frame(), show_rt_xsave_frame(), and
  valid_xsave_frame() to the file crtools.c.

* Introduced the structure fpu_state_t to pass the FPU state to the restorer
  in a machine-independent way.

Signed-off-by: Alexander Kartashov <alekskartashov@parallels.com>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>