As a first pass you might want to create an empty putchar() function just so you can compile and link your program to see if anything else is wrong.

The idea of putchar() is to send a character to the standard output device. In this embedded world, you decide what that device is.

It's up to you to code a useable putchar() to match your hardware and needs. For example, you could create a putchar() that uses a timer interrupt and an LED to flash the character being sent in Morse code. Or use a bicolor LED (red/green) to send the character in binary. Or maybe even an interrupt driven serial transmit function.