arm/restorer: fix {,rt_}sigframe typo

sigreturn_prep_fpu_frame is no-op for arm, but I think it's better
to fix it while macro correctly expands in sigframe.c - it
may change in some future.

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>