target/riscv: Replace Zvbb checking by Zvkb

target/riscv: Replace Zvbb checking by Zvkb

The Zvkb extension is a proper subset of the Zvbb extension and includes
following instructions:
  * vandn.[vv,vx]
  * vbrev8.v
  * vrev8.v
  * vrol.[vv,vx]
  * vror.[vv,vx,vi]

Signed-off-by: Max Chou <max.chou@sifive.com>
Reviewed-by: Daniel Henrique Barboza <dbarboza@ventanamicro.com>
Message-ID: <20231026151828.754279-5-max.chou@sifive.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>