build/sim: use -O0 for verilator compilation

In most of the case, execution speed is already fast enough with -O0 and
with complex design -O0 is a lost faster to compile than -O3. In the future
we could add a switch to choose which optimization we want.

litex/build/sim/core/Makefile

 include variables.mak
 
 CC = gcc
-CFLAGS = -Wall -O3 -ggdb $(if $(COVERAGE), -DVM_COVERAGE)
+CFLAGS = -Wall -O0 -ggdb $(if $(COVERAGE), -DVM_COVERAGE)
 LDFLAGS = -lpthread -ljson-c -lm -lstdc++ -ldl -levent
 
 CC_SRCS ?= "--cc dut.v"