Zcmp Extension

6 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