target/riscv: add support for Zcd extension

target/riscv: add support for Zcd extension

Separate c_fld/c_fsd from fld/fsd to add additional check for
c.fld{sp}/c.fsd{sp} which is useful for zcmp/zcmt to reuse
their encodings.

Signed-off-by: Weiwei Li <liweiwei@iscas.ac.cn>
Signed-off-by: Junqiang Wang <wangjunqiang@iscas.ac.cn>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-Id: <20230307081403.61950-5-liweiwei@iscas.ac.cn>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>