Hello Yasar Arafath,
have you proceeded with errors?
firstly, this post has no connection to the MQX, doesn't have? Second, you did not share any code, therefore let me guess. Undefined reference means definition of the symbol _start is not present in your files, linker does not see it. I would check linker and compiler paths.
This is C language oriented question than MQX :smileyhappy:
Regards,
MartinK