vulkan: optimize mul_mat for small values of N (llama/10991)

vulkan: optimize mul_mat for small values of N (llama/10991)

Make the mul_mat_vec shaders support N>1 (as a spec constant, NUM_COLS) where
the batch_strides are overloaded to hold the row strides. Put the loads from the
B matrix in the innermost loop because it should cache better.

Share some code for reducing the result values to memory in mul_mat_vec_base.