arm: kill fpu_state_t

It has been a mistake to introduce this struct since the FPU state
is stored deep inside an ARM sigframe. A remnant of the struct
is left to make the code compatible with the universal declaration
of the routine sigreturn_prep_fpu_frame().

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