remove useless visibility application from static-linking-only code

part of the goal here is to eliminate use of the ATTR_LIBC_VISIBILITY
macro outside of libc.h, since it was never intended to be 'public'.

src/env/__init_tls.c

@@ -33,7 +33,7 @@ static struct builtin_tls {
 struct tls_image {
 	void *image;
 	size_t len, size, align;
-} __static_tls ATTR_LIBC_VISIBILITY;
+} __static_tls;
 
 #define T __static_tls
 

src/env/__reset_tls.c

@@ -2,12 +2,11 @@
 
 #include <string.h>
 #include "pthread_impl.h"
-#include "libc.h"
 
 extern struct tls_image {
 	void *image;
 	size_t len, size, align;
-} __static_tls ATTR_LIBC_VISIBILITY;
+} __static_tls;
 
 #define T __static_tls