Added pgm_read_word() def for non-AVR chips

Adafruit_GFX.cpp

@@ -39,6 +39,7 @@ POSSIBILITY OF SUCH DAMAGE.
  #include <pgmspace.h>
 #else
  #define pgm_read_byte(addr) (*(const unsigned char *)(addr))
+ #define pgm_read_word(addr) (*(const unsigned short *)(addr))
 #endif
 
 #ifndef min