Add WORD_MSBIT_HIGH define - machine_int_t with the highest bit set.

py/mpconfig.h

@@ -78,6 +78,8 @@
 
 #define BITS_PER_BYTE (8)
 #define BITS_PER_WORD (BITS_PER_BYTE * BYTES_PER_WORD)
+// machine_int_t value with most significant bit set
+#define WORD_MSBIT_HIGH (1 << (BYTES_PER_WORD * 8 - 1))
 
 // printf format spec to use for machine_int_t and friends
 #ifndef INT_FMT