Allow custom verilator optimization flags

sims/verilator/Makefile

@@ -84,7 +84,7 @@ TRACING_CFLAGS := $(if $(filter $(VERILATOR_FST_MODE),0),,-DCY_FST_TRACE)
 #----------------------------------------------------------------------------------------
 # we initially had --noassert for performance, but several modules use
 # assertions, including dramsim, so we enable --assert by default
-VERILATOR_OPT_FLAGS := \
+VERILATOR_OPT_FLAGS ?= \
 	-O3 \
 	--x-assign fast \
 	--x-initial fast \