py: Cleanup use of global DEBUG preprocessor definition

The standard preprocessor definition to differentiate debug and non-debug
builds is NDEBUG, not DEBUG, so don't rely on the latter:
- just delete the use of it in objint_longlong.c as it has been stale code
  for years anyway (since commit [c4029e5]): SUFFIX isn't used anywhere.
- replace DEBUG with MICROPY_DEBUG_NLR in nlr.h: it is rarely used anymore
  so can be off by default

py/nlr.h

@@ -85,7 +85,7 @@ NORETURN void nlr_jump(void *val);
 NORETURN void nlr_jump_fail(void *val);
 
 // use nlr_raise instead of nlr_jump so that debugging is easier
-#ifndef DEBUG
+#ifndef MICROPY_DEBUG_NLR
 #define nlr_raise(val) nlr_jump(MP_OBJ_TO_PTR(val))
 #else
 #include "mpstate.h"