target/riscv: fix shifts shamt value for rv128c

For rv128c shifts, a shamt of 0 is a shamt of 64, while for rv32c/rv64c
it stays 0 and is a hint instruction that does not change processor state.
For rv128c right shifts, the 6-bit shamt is in addition sign extended to
7 bits.

Signed-off-by: Frédéric Pétrot <frederic.petrot@univ-grenoble-alpes.fr>
Reviewed-by: Weiwei Li <liweiwei@iscas.ac.cn>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20220710110451.245567-1-frederic.petrot@univ-grenoble-alpes.fr>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>