Re-enable extern-pre-js and extern-post-js under pthreads

In emscripten-core#21701 the running of extern-pre/post-js in pthreads was disabled.
The reason was that we had a couple of tests that seemed to be assuming
this.  However, it turns out that there are important use cases that
depend on this code running in threads.

Instead, fix the two test cases by adding an extra condition.

Fixes: emscripten-core#22012

test/test_core.py

@@ -9268,7 +9268,7 @@ def test_pthread_offset_converter_modularize(self):
     self.set_setting('USE_OFFSET_CONVERTER')
     self.set_setting('MODULARIZE')
     self.set_setting('EXPORT_NAME', 'foo')
-    create_file('post.js', 'foo();')
+    create_file('post.js', 'if (!isPthread) foo();')
     self.emcc_args += ['--extern-post-js', 'post.js']
     if '-g' in self.emcc_args:
       self.emcc_args += ['-DDEBUG']

test/test_other.py

@@ -366,7 +366,7 @@ def test_emcc_output_mjs(self, args):
   })
   @node_pthreads
   def test_emcc_output_worker_mjs(self, args):
-    create_file('extern-post.js', 'await Module();')
+    create_file('extern-post.js', 'if (!isPthread) await Module();')
     os.mkdir('subdir')
     self.run_process([EMCC, '-o', 'subdir/hello_world.mjs',
                       '-sEXIT_RUNTIME', '-sPROXY_TO_PTHREAD', '-pthread', '-O1',

tools/link.py

@@ -677,13 +677,6 @@ def phase_linker_setup(options, state, newargs):
   options.extern_pre_js = read_js_files(options.extern_pre_js)
   options.extern_post_js = read_js_files(options.extern_post_js)
 
-  if settings.PTHREADS:
-    # Don't run extern pre/post code on pthreads.
-    if options.extern_pre_js:
-      options.extern_pre_js = 'if (!isPthread) {' + options.extern_pre_js + '}'
-    if options.extern_post_js:
-      options.extern_post_js = 'if (!isPthread) {' + options.extern_post_js + '}'
-
   # TODO: support source maps with js_transform
   if options.js_transform and settings.GENERATE_SOURCE_MAP:
     logger.warning('disabling source maps because a js transform is being done')
@@ -2109,8 +2102,6 @@ def phase_final_emitting(options, state, target, wasm_target):
     src = read_file(final_js)
     final_js += '.epp.js'
     with open(final_js, 'w', encoding='utf-8') as f:
-      if settings.PTHREADS and (options.extern_pre_js or options.extern_post_js):
-        f.write(make_pthread_detection())
       f.write(options.extern_pre_js)
       f.write(src)
       f.write(options.extern_post_js)
@@ -2461,8 +2452,7 @@ def modularize(options):
 ''' % {'EXPORT_NAME': settings.EXPORT_NAME}
 
   if settings.PTHREADS:
-    if not options.extern_pre_js and not options.extern_post_js:
-      src += make_pthread_detection()
+    src += make_pthread_detection()
     src += '// When running as a pthread, construct a new instance on startup\n'
     src += 'isPthread && %s();\n' % settings.EXPORT_NAME