net/mlx5: split multi-thread flow handling per OS

net/mlx5: split multi-thread flow handling per OS

multi-threaded flows feature uses pthread function pthread_key_create
but for Windows the destruction option in the function is unimplemented.

To resolve it, Windows will implement destruction mechanism to cleanup
mlx5_flow_workspace object for each terminated thread.

Linux flow will keep the current behavior.

Signed-off-by: Tal Shnaiderman <talshn@nvidia.com>
Acked-by: Matan Azrad <matan@nvidia.com>
Acked-by: Khoa To <khot@microsoft.com>