nptl: More useful padding in struct pthread

The previous use of padding within a union made it impossible to
re-use the padding for GLIBC_PRIVATE ABI preservation because
tcbhead_t could use up all of the padding (as was historically the
case on x86-64).  Allocating padding unconditionally addresses this
issue.

Reviewed-by: Adhemerval Zanella  <adhemerval.zanella@linaro.org>

nptl/descr.h

@@ -128,8 +128,6 @@ struct priority_protection_data
 
 /* Thread descriptor data structure.  */
 struct pthread
-{
-  union
 {
 #if !TLS_DTV_AT_TP
   /* This overlaps the TCB as used for TLS without threads (see tls.h).  */
@@ -157,13 +155,6 @@ struct pthread
   } header;
 #endif
 
-    /* This extra padding has no special purpose, and this structure layout
-       is private and subject to change without affecting the official ABI.
-       We just have it here in case it might be convenient for some
-       implementation-specific instrumentation hack or suchlike.  */
-    void *__padding[24];
-  };
-
   /* This descriptor's link on the GL (dl_stack_used) or
      GL (dl_stack_user) list.  */
   list_t list;
@@ -407,6 +398,9 @@ struct pthread
   /* getrandom vDSO per-thread opaque state.  */
   void *getrandom_buf;
 
+  /* Can be used for backports preserving internal TCB layout.  */
+  void *padding[8];
+
   /* rseq area registered with the kernel.  Use a custom definition
      here to isolate from kernel struct rseq changes.  The
      implementation of sched_getcpu needs acccess to the cpu_id field;