simplify multi-threaded errno, eliminate useless function pointer

src/errno/__errno_location.c

@@ -1,9 +1,8 @@
-#include <errno.h>
-#include "libc.h"
+#include "pthread_impl.h"
 
 int *__errno_location(void)
 {
 	static int e;
-	if (libc.errno_location) return libc.errno_location();
+	if (libc.main_thread) return __pthread_self()->errno_ptr;
 	return &e;
 }