disas/riscv.c: add disasm support for Zc*

Zcmp/Zcmt instructions will override disasm for c.fld*/c.fsd*
instructions currently.

Signed-off-by: Weiwei Li <liweiwei@iscas.ac.cn>
Signed-off-by: Junqiang Wang <wangjunqiang@iscas.ac.cn>
Acked-by: Alistair Francis <alistair.francis@wdc.com>
Message-Id: <20230307081403.61950-10-liweiwei@iscas.ac.cn>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>