py/smallint: Allow to override MP_SMALL_INT_MIN et al.

This allows a port to specify exactly how many bits are in a small
int (eg for a uPy bytecode cross compiler).

py/smallint.h

@@ -31,6 +31,8 @@
 
 // Functions for small integer arithmetic
 
+#ifndef MP_SMALL_INT_MIN
+
 // In SMALL_INT, next-to-highest bits is used as sign, so both must match for value in range
 #if MICROPY_OBJ_REPR == MICROPY_OBJ_REPR_A || MICROPY_OBJ_REPR == MICROPY_OBJ_REPR_C
 
@@ -55,6 +57,8 @@
 
 #endif
 
+#endif
+
 #define MP_SMALL_INT_MAX ((mp_int_t)(~(MP_SMALL_INT_MIN)))
 
 bool mp_small_int_mul_overflow(mp_int_t x, mp_int_t y);