Remove functions already defined in the standard library

Since newlib 2.2.0 itoa() and utoa() are declared in stdlib.h and redefining them makes the compiler unhappy

teensy3/avr_functions.h

 
 char * ultoa(unsigned long val, char *buf, int radix);
 char * ltoa(long val, char *buf, int radix);
+
+#if defined(_NEWLIB_VERSION) && (__NEWLIB__ < 2 || __NEWLIB__ == 2 && __NEWLIB_MINOR__ < 2)
 static inline char * utoa(unsigned int val, char *buf, int radix) __attribute__((always_inline, unused));
 static inline char * utoa(unsigned int val, char *buf, int radix) { return ultoa(val, buf, radix); }
 static inline char * itoa(int val, char *buf, int radix) __attribute__((always_inline, unused));
 static inline char * itoa(int val, char *buf, int radix) { return ltoa(val, buf, radix); }
+#endif
+
 char * dtostrf(float val, int width, unsigned int precision, char *buf);