aarch64-linux-user: Add support for SVE signal frame records

Depending on the currently selected size of the SVE vector registers,
we can either store the data within the "standard" allocation, or we
may beedn to allocate additional space with an EXTRA record.

Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20180303143823.27055-6-richard.henderson@linaro.org
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>