Zcmp Extension

6 instructions and 0 pseudo-instructions in the Zcmp extension.

• cm.mva01s - Move two s0-s7 registers into a0-a1
• cm.mvsa01 - Move a0-a1 into two registers of s0-s7
• cm.pop - Destroy function call stack frame
• cm.popret - Destroy function call stack frame and return to `ra`
• cm.popretz - Destroy function call stack frame, move zero to `a0` and return to `ra`
• cm.push - Create function call stack frame