unix: Add CFLAGS_EXTRA & LDFLAGS_EXTRA for command line usage.

The idea is that it should be possible to pass any additional params for
experimentation without need to patch sources (and without need to deviate
from or repeat baseline options).

unix/Makefile

@@ -19,7 +19,7 @@ INC += -I$(BUILD)
 
 # compiler settings
 CWARN = -Wall -Werror
-CFLAGS = $(INC) $(CWARN) -ansi -std=gnu99 -DUNIX $(CFLAGS_MOD) $(COPT)
+CFLAGS = $(INC) $(CWARN) -ansi -std=gnu99 -DUNIX $(CFLAGS_MOD) $(COPT) $(CFLAGS_EXTRA)
 
 # Debugging/Optimization
 ifdef DEBUG
@@ -29,7 +29,7 @@ else
 COPT = -Os #-DNDEBUG
 endif
 
-LDFLAGS = $(LDFLAGS_MOD) -lm -Wl,-Map=$@.map,--cref
+LDFLAGS = $(LDFLAGS_MOD) -lm -Wl,-Map=$@.map,--cref $(LDFLAGS_EXTRA)
 
 ifeq ($(MICROPY_FORCE_32BIT),1)
 CFLAGS += -m32