Review: ecall and ebreak take rd register selector

docs/pages/specs/nexus-vm.mdx

@@ -85,8 +85,8 @@ Table: Summary of the Nexus Virtual Machine Instruction Set, where operations ar
 | $\textbf{jal}$ | $rd$ $i$ | jumps to $pc+i$  and stores $pc+4$ into $R[rd]$|
 | $\textbf{jalr}$ | $rd$ $rs_1$ $i$ | jumps to $R[rs_1] + i$ and stores $pc+4$ into $R[rd]$|
 | $\textbf{fence}$ |  | No operation |
-| $\textbf{ecall}$ |  | system call |
-| $\textbf{ebreak}$ |  | system call|
+| $\textbf{ecall}$ | $rd$ | system call |
+| $\textbf{ebreak}$ | $rd$  | system call|
 | $\textbf{unimp}$ |  | jumps to $pc$; in effect looping forever at the current program counter |
 
 The Nexus VM also enforces 2-byte and 4-byte memory alignments for the instructions operating on half-words and words.