Avoid use of __EMSCRIPTEN_PTHREADS__ in __errno_location.c

Instead we can use TLS which works for both single and multithreaded
builds and also works with wasm workers.

This is also needed for #22735

system/lib/libc/musl/src/errno/__errno_location.c

@@ -1,22 +1,19 @@
 #include <errno.h>
 #include "pthread_impl.h"
 
-#if __EMSCRIPTEN_PTHREADS__
-// for pthreads, use the proper location on the thread info, so each
-// thread has its own errno
-int *__errno_location(void)
-{
-	return &__pthread_self()->errno_val;
-}
-#else
-// for single-threaded mode, avoid linking in pthreads support code
-// just for this
-static int __errno_storage = 0;
+#if __EMSCRIPTEN__
+// For emscripten we use TLS here instead of `__pthread_self`, so that in single
+// threaded builds this get lowered away to normal global variable.
+static _Thread_local int __errno_storage = 0;
+#endif
 
 int *__errno_location(void)
 {
+#if __EMSCRIPTEN__
 	return &__errno_storage;
-}
+#else
+	return &__pthread_self()->errno_val;
 #endif
+}
 
 weak_alias(__errno_location, ___errno_location);

test/code_size/hello_wasm_worker_wasm.json

@@ -5,8 +5,8 @@
   "a.js.gz": 455,
   "a.ww.js": 115,
   "a.ww.js.gz": 127,
-  "a.wasm": 1850,
-  "a.wasm.gz": 1050,
-  "total": 3248,
-  "total_gz": 2016
+  "a.wasm": 1894,
+  "a.wasm.gz": 1077,
+  "total": 3292,
+  "total_gz": 2043
 }

test/test_sockets.py

@@ -328,7 +328,7 @@ def test_nodejs_sockets_echo_subprotocol(self):
   # NOTE: Shared buffer is not allowed for websocket sending.
   @parameterized({
     '': [[]],
-    'shared': [['-sSHARED_MEMORY']],
+    'pthread': [['-pthread']],
   })
   def test_websocket_send(self, args):
     with NodeJsWebSocketEchoServerProcess():