Add micro-kernel to compute FP16 GEMV

* Compute the general matrix-vector (GEMV) multiplication between an FP16 LHS and RHS and accumulate into FP16 output. The RHS packs FP16 weights and biases together.

* Optimized for Arm® Neon™ using MLA instructions.

* Add accompanying tests.

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

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