Issue illegal instruction for deconfigured B extensions

[silabs-oivind]

Illegal instruction won't be issued from the B decoder if trying to execute a deconfigured instruction.

In the example below (and other similar ones), add "else decoder_ctrl_o = DECODER_CTRL_ILLEGAL_INSN":

     {7'b001_0000, 3'b010}: begin // Shift left by 1 and add (sh1add)
        if (RV32B_ZBA) begin
          decoder_ctrl_o.alu_operator = ALU_B_SH1ADD;
        end
      end

[Silabs-ArjanB]

Also put the carryless multiplication circuit in the ALU in a generate statement as it it not obvious that it will get optimized away if not used.

[Silabs-ArjanB]

@silabs-oivind Please close this issue if you agree it has been solved.