Contain SME2 assembly inside SMSTART/SMSTOP boundary

Regenerate the SME2 GEMV micro-kernel assembly so that it is contained within the SMSTART/SMSTOP boundary, preventing illegal instruction faults when attempting to execute streaming SVE code on a system without SVE support.

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

Approved-by: Emil Ohlsson <emil.ohlsson@arm.com>