대략 아래와 같이 하시면 될 겁니다.
include $(CLEAR_VARS)
LOCAL_MODULE := A
LOCAL_SRC_FILES := A.a
include $(PREBUILT_STATIC_LIBRARY)
include $(CLEAR_VARS)
LOCAL_MODULE := B
LOCAL_SRC_FILES := B.a
include $(PREBUILT_STATIC_LIBRARY)
include $(CLEAR_VARS)
LOCAL_STATIC_LIBRARIES := A B
LOCAL_MODULE := C
include $(BUILD_STATIC_LIBRARY)