Exit from testbench, not C code

Otherwise, we don't get coverage data from the simulator.

vsim/Makefrag

@@ -41,6 +41,7 @@ VCS_OPTS = -notice -line +lint=all,noVCDE,noONGS,noUI -error=PCWM-L -timescale=1
 	-e vcs_main \
 	$(RISCV)/lib/libfesvr.so \
 	$(sim_dir)/libdramsim.a \
+	-sverilog \
 	+incdir+$(generated_dir) \
 	+define+CLOCK_PERIOD=0.5 $(sim_vsrcs) $(sim_csrcs) \
 	+define+PRINTF_COND=$(TB).printf_cond \