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

The EXTRA record allows for additional space to be allocated
beyon what is currently reserved.  Add code to emit and read
this record type.

Nothing uses extra space yet.

Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20180303143823.27055-5-richard.henderson@linaro.org
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>