Add FP16 SME kernels

Add fp16 kernels for LHS and RHS packing, and matmul. Also add related
unit tests for said kernels, and extend unit Matmul tests to support
calling fp16 kernels.

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

Reviewed-by: Viet-Hoa Do <viet-hoa.do@arm.com>
Approved-by: Felix Johnny Thomasmathibalan <felixjohnny.thomasmathibalan@arm.com>