sve: Fix signal frame z/v register restore

While doing some SME work, I ran into the situation where the Z register
contents restored from a signal frame are incorrect if the signal frame
only contains fpsimd state and no sve state.

This happens because we only restore the v register values in that case,
and don't do anything for the z registers.

Fix this by initializing the z registers to 0 and then copying over the
overlapping part of the v registers to the z registers.

While at it, refactor the code a bit to simplify it and make it smaller.

Regression-tested on aarch64-linux Ubuntu 22.04/20.04.

Reviewed-by: Thiago Jung Bauermann <thiago.bauermann@linaro.org>