Read SVL without entering Streaming SVE mode

The RDSVL instruction allows for reading the effective Streaming SVE Vector Length (SVL) without having the Processing Element enter Streaming SVE mode.

Signed-off-by: Jakub Sujak <jakub.sujak@arm.com>

Reviewed-by: Viet-Hoa Do <viet-hoa.do@arm.com>
Approved-by: Viet-Hoa Do <viet-hoa.do@arm.com>