[sw/lib] add nerov32-flavored vprintf funtion

[stnolting]

Suggested by @jpf91 in #710

[jpf91]

Great, thanks! Now it's really easy to write a task-safe wrapper, e.g. for FreeRTOS:

void uart_printf_locked(neorv32_uart_t *UARTx, const char *format, ...) {

  va_list args;
  va_start(args, format);
  xSemaphoreTake(...)
  neorv32_uart_vprintf(UARTx, format, args);
  xSemaphoreGive(...)
  va_end(args);
}