Zvk: Implement Zvkg, Vector GCM/GMAC instruction

Implement the proposed instruction in Zvkg, vghmac.vv,
Vector Carryless Multiply Accumulate over GHASH Galois-Field.

The instruction performs one step of GHASH routine as described
in "NIST Special Publication 800-38D" a.k.a the AES-GCM specification.
The logic was written to closely track the pseudo-code
in the Zvk specification.

Signed-off-by: Eric Gouriou <ego@rivosinc.com>
Co-authored-by: Kornel Duleba <mindal@semihalf.com>
Signed-off-by: Eric Gouriou <ego@rivosinc.com>