risugen: go back to storing 8192 bytes of random data

go back to store 8192 bytes of random data,
even though we actually use only the first 2K.

Signed-off-by: Claudio Fontana <claudio.fontana@linaro.org>
risu_reginfo_aarch64: add FP/SIMD state

initialize and check FP/SIMD state.

Signed-off-by: Claudio Fontana <claudio.fontana@linaro.org>