Add EXTRA_SIM_PREPROC_DEFINES to common-sims-flags.mk

This allows adding e.g. EXTRA_SIM_PREPROC_DEFINES="+define+NUM_CORES=2"
to the make command to change Vortex Verilog parameters.

sims/common-sim-flags.mk

@@ -46,4 +46,5 @@ SIM_PREPROC_DEFINES = \
 	+define+RANDOMIZE_MEM_INIT \
 	+define+RANDOMIZE_REG_INIT \
 	+define+RANDOMIZE_GARBAGE_ASSIGN \
-	+define+RANDOMIZE_INVALID_ASSIGN
+	+define+RANDOMIZE_INVALID_ASSIGN \
+	$(EXTRA_SIM_PREPROC_DEFINES)