RISC-V: Add missing DECLARE_INSNs for Zicbo{m,p,z}

The recently added support for the Zicbo{m,p,z} extensions did not
include DECLARE_INSN() declarations for the instructions.
These declarations are needed by GDB's instruction detection code.
This patch adds them.

Signed-off-by: Christoph Muellner <cmuellner@gcc.gnu.org>