Fix lem build error

The speculate_conditional should be marked monadic. It would seem like
the various _reservation functions should be also, but it seems like
perhaps they are not implemented in lem right now.

model/riscv_sys_control.sail

@@ -228,7 +228,7 @@ function check_CSR(csr : csreg, p : Privilege, isWrite : bool) -> bool =
  * where cancellation can be performed.
  */
 
-val speculate_conditional = {ocaml: "Platform.speculate_conditional", interpreter: "excl_res", c: "speculate_conditional", lem: "speculate_conditional_success"} : unit -> bool
+val speculate_conditional = monadic {ocaml: "Platform.speculate_conditional", interpreter: "excl_res", c: "speculate_conditional", lem: "speculate_conditional_success"} : unit -> bool
 
 val load_reservation = {ocaml: "Platform.load_reservation", interpreter: "Platform.load_reservation", c: "load_reservation", lem: "load_reservation"} : xlenbits -> unit
 val match_reservation = {ocaml: "Platform.match_reservation", interpreter: "Platform.match_reservation", lem: "match_reservation", c: "match_reservation"} : xlenbits -> bool