elf: Apply attribute_relro to pointers in elf/dl-minimal.c

The present code leaves the function pointers unprotected, but moves
some of the static functions into .data.rel.ro instead.  This causes
the linker to produce an allocatable, executable, writable section
and eventually an RWX load segment.  Not only do we really do not
want that, it also breaks valgrind because valgrind does not load
debuginfo from the mmap interceptor if all it sees are RX and RWX
mappings.

Fixes commit 3a0ecccb59 ("ld.so: Do not
export free/calloc/malloc/realloc functions [BZ #25486]").

elf/dl-minimal.c

@@ -39,16 +39,16 @@
   implementation below.  Before the final relocation,
   __rtld_malloc_init_real is called to replace the pointers with the
   real implementation.  */
-__typeof (calloc) *__rtld_calloc;
-__typeof (free) *__rtld_free;
-__typeof (malloc) *__rtld_malloc;
-__typeof (realloc) *__rtld_realloc;
+__typeof (calloc) *__rtld_calloc attribute_relro;
+__typeof (free) *__rtld_free attribute_relro;
+__typeof (malloc) *__rtld_malloc attribute_relro;
+__typeof (realloc) *__rtld_realloc attribute_relro;
 
 /* Defined below.  */
-static __typeof (calloc) rtld_calloc attribute_relro;
-static __typeof (free) rtld_free attribute_relro;
-static __typeof (malloc) rtld_malloc attribute_relro;
-static __typeof (realloc) rtld_realloc attribute_relro;
+static __typeof (calloc) rtld_calloc;
+static __typeof (free) rtld_free;
+static __typeof (malloc) rtld_malloc;
+static __typeof (realloc) rtld_realloc;
 
 void
 __rtld_malloc_init_stubs (void)