arch: arm: save 'fpscr' in the svc and isr handler

`fpscr` is assigned from `struct __fpu_sf.fpscr` in `vfp_restore`, but it
wasn't saved into `struct __fpu_sf.fpscr` in the svc and isr handler, So
it may be a dirty value.

    - Fix it by saving `fpscr` in the svc hand isr handler.
    - Jump out if FPU isn't enabled

Signed-off-by: Huifeng Zhang <Huifeng.Zhang@arm.com>