Force link _printf_float in dtostrf

cores/arduino/avr/dtostrf.c

@@ -20,6 +20,8 @@
 #include <stdio.h>
 
 char *dtostrf (double val, signed char width, unsigned char prec, char *sout) {
+  asm(".global _printf_float");
+
   char fmt[20];
   sprintf(fmt, "%%%d.%df", width, prec);
   sprintf(sout, fmt, val);