Fix tracer during random gnt and valid stalls

Created by: davideschiavone

When simulating the example_tb with modelsim with random stalls on Data and Instruction memory as:

make custom-vsim-run CUSTOM_GCC_FLAGS="-DRANDOM_MEM_STALL" I get the errors:

UVM_ERROR ../../bhv/cv32e40p_tracer.sv(443) @ 3085ns: reporter [TRACER] WB .....

I imported from the core-v-verif the modules to create such stalls as suggested in #623

Plus, I updated the Makefile to support xpulp instructions

Signed-off-by: Pasquale Davide Schiavone davide@openhwgroup.org