getting mass storage example to work using J-link

You need a #pragma to stop in including library stuff in the linker,

/**************************************************************************/

/* Implementation of putchar (also used by printf function to output data)    */
int SendChar(int ch)                    /* Write character to Serial Port     */
{
        ITM_SendChar(ch); // From core_cm4.c

  return(ch);
}

/**************************************************************************/

#include <rt_misc.h>

#pragma import(__use_no_semihosting_swi)

struct __FILE { int handle; /* Add whatever you need here */ };
FILE __stdout;

int fputc (int ch, FILE *f) { return (SendChar(ch)); }

int ferror (FILE *f) {
  /* Your implementation of ferror */
  return EOF;
}

void _ttywrch (int ch) { SendChar(ch); }

void _sys_exit (int return_code) { for (;;); }

/**************************************************************************/

You'll need to #include <stdio.h> and make sure you have the trace clock matching the system clock. There are other retargeting examples, you might want to check those and the documentation.