unix/Makefile: Use -Og instead of -O0 for debug builds.

For the coverage build this reduces the binary size to about 1/4 of its
size, and seems to help gcov/lcov coverage analysis so that it doesn't miss
lines.

Signed-off-by: Damien George <damien@micropython.org>

ports/unix/Makefile

@@ -44,7 +44,7 @@ CFLAGS += $(INC) $(CWARN) -std=gnu99 -DUNIX $(CFLAGS_MOD) $(COPT) -I$(VARIANT_DI
 
 # Debugging/Optimization
 ifdef DEBUG
-COPT ?= -O0
+COPT ?= -Og
 else
 COPT ?= -Os
 COPT += -DNDEBUG