Don't do memory operations in pthreads before the runtime is initialized. NFC (#12052)

Pthreads.initMainThreadBlock is called during "preRun" (before compiled code is ready) and
Pthreads.initRuntime is called during "atInit" (when compiled code is ready, just before
global ctors). This moves some code from the former into the latter. Specifically it moves
all allocation and memory operations.

This leaves initMainThreadBlock to only do pure JS operations for preloading the workers,
and everything else to initRuntime.

Author: kripken

src/library_pthread.js

@@ -23,12 +23,6 @@ var LibraryPThread = {
     runningWorkers: [],
     // Points to a pthread_t structure in the Emscripten main heap, allocated on demand if/when first needed.
     // mainThreadBlock: undefined,
-    initRuntime: function() {
-      // Pass the thread address inside the asm.js scope to store it for fast access that avoids the need for a FFI out.
-      // Global constructors trying to access this value will read the wrong value, but that is UB anyway.
-      registerPthreadPtr(PThread.mainThreadBlock, /*isMainBrowserThread=*/!ENVIRONMENT_IS_WORKER, /*isMainRuntimeThread=*/1);
-      _emscripten_register_main_browser_thread_id(PThread.mainThreadBlock);
-    },
     initMainThreadBlock: function() {
 #if ASSERTIONS
       assert(!ENVIRONMENT_IS_PTHREAD);
@@ -41,7 +35,8 @@ var LibraryPThread = {
         PThread.allocateUnusedWorker();
       }
 #endif
-
+    },
+    initRuntime: function() {
       PThread.mainThreadBlock = {{{ makeStaticAlloc(C_STRUCTS.pthread.__size__) }}};
 
       for (var i = 0; i < {{{ C_STRUCTS.pthread.__size__ }}}/4; ++i) HEAPU32[PThread.mainThreadBlock/4+i] = 0;
@@ -66,6 +61,11 @@ var LibraryPThread = {
       PThread.setThreadName(PThread.mainThreadBlock, "Browser main thread");
       PThread.setThreadStatus(PThread.mainThreadBlock, {{{ cDefine('EM_THREAD_STATUS_RUNNING') }}});
 #endif
+
+      // Pass the thread address inside the asm.js scope to store it for fast access that avoids the need for a FFI out.
+      // Global constructors trying to access this value will read the wrong value, but that is UB anyway.
+      registerPthreadPtr(PThread.mainThreadBlock, /*isMainBrowserThread=*/!ENVIRONMENT_IS_WORKER, /*isMainRuntimeThread=*/1);
+      _emscripten_register_main_browser_thread_id(PThread.mainThreadBlock);
     },
     initWorker: function() {
 #if USE_CLOSURE_COMPILER