[JSON] Suppress command echo to stdout

Currently, when executing `make json`, the command itself is echoed,
causing issues with the JSON representation.

This PR Suppress this echoing, ensuring a proper JSON output
without the need for any manual editing later on.

Makefile

@@ -200,7 +200,7 @@ all: ocaml_emulator/riscv_ocaml_sim_$(ARCH) c_emulator/riscv_sim_$(ARCH) riscv_i
 .PHONY: all
 
 json: $(SAIL_SRCS) model/main.sail Makefile
-	sail -json $(SAIL_FLAGS) $(SAIL_SRCS)
+	@sail -json $(SAIL_FLAGS) $(SAIL_SRCS)
 
 output: $(SAIL_SRCS) model/main.sail Makefile
 	sail -output-sail $(SAIL_FLAGS) $(SAIL_SRCS)