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/thread/pthread_create.c

@@ -201,7 +201,6 @@ int pthread_create(pthread_t *restrict res, const pthread_attr_t *restrict attrp
 	new->stack = stack;
 	new->stack_size = stack - stack_limit;
 	new->pid = self->pid;
-	new->errno_ptr = &new->errno_val;
 	new->start = entry;
 	new->start_arg = arg;
 	new->self = new;