По рекоменндациям из интерннета написал такую функцию

_C_LIB_DECL 
int putchar(int ch)
{
  while ( !(U3LSR & LSR_THRE) );	

  if (ch == '\n') U3THR = CR;
  else U3THR = ch;
  return 1;
}
_END_C_LIB_DECL

Получаю ошибку

..\common\DBGU\dbgu.c(29): error:  #77-D: this declaration has no storage class or type specifier
..\common\DBGU\dbgu.c(30): error:  #65: expected a ";"
..\common\DBGU\dbgu.c(38): warning: At end of source:  #12-D: parsing restarts here after previous syntax error

По другой рекомендации создал файл Retarget.c

#pragma import(__use_no_semihosting_swi)

int putchar(int ch)
{
  while ( !(U3LSR & LSR_THRE) );	

  if (ch == '\n') U3THR = CR;
  else U3THR = ch;
  return 1;
}

Получаю

.\Objects\easyweb.axf: Error: L6915E: Library reports error: __use_no_semihosting_swi was requested, but _sys_open was referenced