Simulator: add pgm_read_dword() macro.

pgm_read_word() was already defined, but pgm_read_dword() did not
get in our way during earlier testing. These functions force the
arduino to read these "defined in program" constant arrays from
"program memory" instead of RAM as a means to save on RAM usage.
The PC-based simulator doesn't need such tricks and only needs to
read the value directly from the in-RAM array. Therefore it is safe
to convert read directly from the pointer being passed.

pgm_read_word() already does this. Define pgm_read_dword()
similarly to it. Fixes #125.

simulator.h

@@ -59,6 +59,7 @@
 #define PSTR(x) (x)
 #define pgm_read_byte(x) (*((uint8_t *)(x)))
 #define pgm_read_word(x) (*((uint16_t *)(x)))
+#define pgm_read_dword(x) (*((uint32_t *)(x)))
 
 #define MASK(PIN)   (1 << PIN)
 #define ACD         7