net/ixgbe: add vector stubs for RISC-V

net/ixgbe: add vector stubs for RISC-V

Re-use vector processing stubs in ixgbe PMD defined for PPC for RISC-V.
This enables ixgbe PMD usage in scalar mode on this architecture.

The ixgbe PMD driver was validated with Intel X520-DA2 NIC and the
test-pmd application. Packet transfer checked using all UIO drivers
available for non-IOMMU platforms: uio_pci_generic, vfio-pci noiommu and
igb_uio.

Sponsored-by: Frank Zhao <frank.zhao@starfivetech.com>
Sponsored-by: Sam Grove <sam.grove@sifive.com>
Signed-off-by: Stanislaw Kardach <kda@semihalf.com>