Fix other arch build in detect.

When CORE is empty, use -march=loongson3a. Fix it.

Signed-off-by: Han Gao <gaohan@uniontech.com>

Makefile.system

@@ -847,7 +847,7 @@ CCOMMON_OPT += -mabi=32
 BINARY_DEFINED = 1
 endif
 
-ifeq ($(CORE), $(filter $(CORE),LOONGSON3R3 LOONGSON3R4))
+ifneq (, $(filter $(CORE),LOONGSON3R3 LOONGSON3R4))
 CCOMMON_OPT += -march=loongson3a
 FCOMMON_OPT += -march=loongson3a
 endif