Fix __errno_location under wasm workers. NFC

Previously the wasm workers build of `__errno_location` was using the
single threaded code path which doesn't use a thead local location.

In addition this change avoids the use of `__EMSCRIPTEN_PTHREADS__` in
`__errno_location.c` which is helpful for unifying the shared memory
libc build.  See emscripten-core#22735.

Author: sbc100

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 gets 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
 }

tools/system_libs.py

@@ -742,7 +742,7 @@ def vary_on(cls):
 
   @classmethod
   def get_default_variation(cls, **kwargs):
-    return super().get_default_variation(is_mt=settings.PTHREADS, is_ww=settings.WASM_WORKERS and not settings.PTHREADS, **kwargs)
+    return super().get_default_variation(is_mt=settings.PTHREADS, is_ww=settings.SHARED_MEMORY and not settings.PTHREADS, **kwargs)
 
   @classmethod
   def variations(cls):