py: Implement sys.maxsize, standard way to check platform "bitness".

Implementing it as a static constant is a bit peculiar and require cooperation
from long int implementation.
diff --git a/py/mpconfig.h b/py/mpconfig.h
index da9cffc..c04c69c 100644
--- a/py/mpconfig.h
+++ b/py/mpconfig.h
@@ -336,6 +336,11 @@
 #define MICROPY_PY_SYS (1)
 #endif
 
+// Whether to provide "sys.maxsize" constant
+#ifndef MICROPY_PY_SYS_MAXSIZE
+#define MICROPY_PY_SYS_MAXSIZE (0)
+#endif
+
 // Whether to provide "sys.exit" function
 #ifndef MICROPY_PY_SYS_EXIT
 #define MICROPY_PY_SYS_EXIT (0)