Only run --no-exit-runtime when optimizing (#11998)

That pass is a pure optimization: it removes calls to atexit when they
would be ignored anyhow (EXIT_RUNTIME == 0).

This removes a warning in atexit's implementation that was never
actually called before: we used to always run that pass, so if
EXIT_RUNTIME == 0 then we never had any calls to atexit
anyhow. Now that we only run the pass when optimizing, leaving
that warning would be a noticeable change (and it broke some
tests actually!) so just remove it. With that, this is essentially
NFC except that non-optimized builds may be a little larger
(containing calls to atexit that end up doing nothing).

Helps WebAssembly/binaryen#3043

Author: kripken

emcc.py

@@ -589,7 +589,7 @@ def backend_binaryen_passes():
   if shared.Settings.SAFE_HEAP:
     passes += ['--safe-heap']
   passes += ['--post-emscripten']
-  if not shared.Settings.EXIT_RUNTIME:
+  if shared.Settings.OPT_LEVEL > 0 and not shared.Settings.EXIT_RUNTIME:
     passes += ['--no-exit-runtime']
   if shared.Settings.OPT_LEVEL > 0 or shared.Settings.SHRINK_LEVEL > 0:
     passes += [building.opt_level_to_str(shared.Settings.OPT_LEVEL, shared.Settings.SHRINK_LEVEL)]

src/library.js

@@ -709,12 +709,6 @@ LibraryManager.library = {
   atexit__proxy: 'sync',
   atexit__sig: 'iii',
   atexit: function(func, arg) {
-#if ASSERTIONS
-#if EXIT_RUNTIME == 0
-    warnOnce('atexit() called, but EXIT_RUNTIME is not set, so atexits() will not be called. set EXIT_RUNTIME to 1 (see the FAQ)');
-#endif
-#endif
-
 #if EXIT_RUNTIME
     __ATEXIT__.unshift({ func: func, arg: arg });
 #endif