elf: Avoid using memalign for TLS allocations [BZ #17730]

Instead of a flag which indicates the pointer can be freed, dtv_t
now includes the pointer which should be freed.  Due to padding,
the size of dtv_t does not increase.

To avoid using memalign, the new allocate_dtv_entry function
allocates a sufficiently large buffer so that a sub-buffer
can be found in it which starts with an aligned pointer.  Both
the aligned and original pointers are kept, the latter for calling
free later.

csu/libc-tls.c

@@ -175,7 +175,7 @@ __libc_setup_tls (size_t tcbsize, size_t tcbalign)
 #else
 # error "Either TLS_TCB_AT_TP or TLS_DTV_AT_TP must be defined"
 #endif
-  _dl_static_dtv[2].pointer.is_static = true;
+  _dl_static_dtv[2].pointer.to_free = NULL;
   /* sbrk gives us zero'd memory, so we don't need to clear the remainder.  */
   memcpy (_dl_static_dtv[2].pointer.val, initimage, filesz);