unix: Add "coverage" target to do coverage testing using gcov.

unix/Makefile

@@ -131,3 +131,11 @@ fast:
 minimal:
 	@echo Make sure to run make -B
 	$(MAKE) COPT="-Os -DNDEBUG" CFLAGS_EXTRA='-DMP_CONFIGFILE="<mpconfigport_minimal.h>"' BUILD=build-minimal PROG=micropython_minimal MICROPY_PY_TIME=0 MICROPY_PY_TERMIOS=0 MICROPY_PY_SOCKET=0 MICROPY_PY_FFI=0
+
+# build an interpreter for coverage testing and do the testing
+coverage:
+	@echo Make sure to run make -B
+	$(MAKE) COPT="-O0" CFLAGS_EXTRA='-fprofile-arcs -ftest-coverage' LDFLAGS_EXTRA='-fprofile-arcs -ftest-coverage' BUILD=build-coverage PROG=micropython_coverage
+	$(eval DIRNAME=$(notdir $(CURDIR)))
+	cd ../tests && MICROPY_MICROPYTHON=../$(DIRNAME)/micropython_coverage ./run-tests
+	gcov -o build-coverage/py ../py/*.c