simplify errno implementation

the motivation for the errno_ptr field in the thread structure, which
this commit removes, was to allow the main thread's errno to keep its
address when lazy thread pointer initialization was used. &errno was
evaluated prior to setting up the thread pointer and stored in
errno_ptr for the main thread; subsequently created threads would have
errno_ptr pointing to their own errno_val in the thread structure.

since lazy initialization was removed, there is no need for this extra
level of indirection; __errno_location can simply return the address
of the thread's errno_val directly. this does cause &errno to change,
but the change happens before entry to application code, and thus is
not observable.

src/errno/__errno_location.c

@@ -3,6 +3,6 @@
 int *__errno_location(void)
 {
 	static int e;
-	if (libc.has_thread_pointer) return __pthread_self()->errno_ptr;
+	if (libc.has_thread_pointer) return &__pthread_self()->errno_val;
 	return &e;
 }