Makefile.emu: do not use --threads 1, use --threads 0 instead

* --threads 1 will create a multi-threaded model but with only one
  thread, which is unnecessary and will reduce performance

Makefile.emu

@@ -17,8 +17,8 @@ VEXTRA_FLAGS += --trace
 endif
 
 # Verilator multi-thread support
-EMU_THREADS  ?= 1
-ifneq ($(EMU_THREADS),1)
+EMU_THREADS  ?= 0
+ifneq ($(EMU_THREADS),0)
 VEXTRA_FLAGS += --threads $(EMU_THREADS) --threads-dpi all
 endif