Fix -Oz + -sLINKABLE (#17194)

This was broken by the recent change to the way we include libc_optz.
See #17179.

Author: sbc100

src/library.js

@@ -368,7 +368,7 @@ mergeInto(LibraryManager.library, {
   // end up adding code that refers to this.)
   // In STANDALONE_WASM we avoid the emscripten_memcpy_big dependency so keep
   // the wasm file standalone.
-#if (SHRINK_LEVEL < 2 || MAIN_MODULE == 1) && !STANDALONE_WASM
+#if (SHRINK_LEVEL < 2 || LINKABLE) && !STANDALONE_WASM
 
   emscripten_memcpy_big__sig: 'vppp',
 #if MIN_CHROME_VERSION < 45 || MIN_EDGE_VERSION < 14 || MIN_FIREFOX_VERSION < 34 || MIN_IE_VERSION != TARGET_NOT_SUPPORTED || MIN_SAFARI_VERSION < 100101

tests/test_other.py

@@ -1249,7 +1249,9 @@ def test_export_all(self):
       };
     ''')
 
-    self.emcc('lib.c', ['-sEXPORT_ALL', '-sLINKABLE', '--pre-js', 'main.js'], output_filename='a.out.js')
+    # Explicitly test with -Oz to ensure libc_optz is included alongside
+    # libc when `--whole-archive` is used.
+    self.emcc('lib.c', ['-Oz', '-sEXPORT_ALL', '-sLINKABLE', '--pre-js', 'main.js'], output_filename='a.out.js')
     self.assertContained('libf1\nlibf2\n', self.run_js('a.out.js'))
 
   def test_export_all_and_exported_functions(self):

tools/system_libs.py

@@ -1861,12 +1861,13 @@ def add_library(libname):
   # C libraries that override libc must come before it
   if settings.PRINTF_LONG_DOUBLE:
     add_library('libprintf_long_double')
-  # libc_optz is a size optimization, and therefore not really important when
-  # MAIN_MODULE=1 (which links in all system libraries, leading to a large
-  # size far bigger than any savings from libc_optz). Also, libc_optz overrides
-  # parts of libc, which will not link due to --whole-archive in MAIN_MODULE=1
-  # currently.
-  if settings.SHRINK_LEVEL >= 2 and settings.MAIN_MODULE != 1:
+  # Becuase libc_optz overrides parts of libc, it is not compatible with `LINKABLE`
+  # (used in `MAIN_MODULE=1`) because with that setting we use `--whole-archive` to
+  # include all system libraries.
+  # However, because libc_optz is a size optimization it is not really important
+  # when used with `MAIN_MODULE=1` (which links in all system libraries, leading
+  # to overheads far bigger than any savings from libc_optz)
+  if settings.SHRINK_LEVEL >= 2 and not settings.LINKABLE:
     add_library('libc_optz')
 
   if settings.STANDALONE_WASM: