target/riscv: Add instructions of the Zbc-extension

target/riscv: Add instructions of the Zbc-extension

The following instructions are part of Zbc:
 - clmul
 - clmulh
 - clmulr

Note that these instructions were already defined in the pre-0.93 and
the 0.93 draft-B proposals, but had not been omitted in the earlier
addition of draft-B to QEmu.

Signed-off-by: Philipp Tomsich <philipp.tomsich@vrull.eu>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-id: 20210911140016.834071-10-philipp.tomsich@vrull.eu
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>