Make _Thread_local work across platforms

We now rewrite the binary image at runtime on Windows and XNU to change
mov %fs:0,%reg instructions to use %gs instead. There's also simpler
threading API introduced by this change and it's called _spawn() and
_join(), which has replaced most clone() usage.

third_party/python/Objects/obmalloc.c

@@ -184,7 +184,7 @@ static void *
 _PyObject_ArenaMmap(void *ctx, size_t size)
 {
 #ifdef __COSMOPOLITAN__
-    return mapanon(size);
+    return _mapanon(size);
 #else
     void *ptr;
     ptr = mmap(NULL, size, PROT_READ|PROT_WRITE,