Linux/x86: Update TARGET_THREAD_SPLIT_STACK_OFFSET comments

Linux/x86: Update TARGET_THREAD_SPLIT_STACK_OFFSET comments

Glibc 2.18 was changed by

commit ecbf434213c0333d81706074e4d107ac45011635
Author: Andreas Jaeger <aj@suse.de>
Date:   Wed May 15 20:20:54 2013 +0200

    Reserve new TLS field for x86 and x86_64

        [BZ #10686]
        * sysdeps/x86_64/tls.h (struct tcbhead_t): Add __private_ss
        field.
        * sysdeps/i386/tls.h (struct tcbhead_t): Likewise.

to reduce the size of __private_tm to make room for __private_ss, which
was supposed to be used for TARGET_THREAD_SPLIT_STACK_OFFSET:

typedef struct
{
  void *tcb;            /* Pointer to the TCB.  Not necessarily the
                           thread descriptor used by libpthread.  */
  dtv_t *dtv;
  void *self;           /* Pointer to the thread descriptor.  */
  int multiple_threads;
  uintptr_t sysinfo;
  uintptr_t stack_guard;
  uintptr_t pointer_guard;
  int gscope_flag;
  int __glibc_reserved1;
  /* Reservation of some values for the TM ABI.  */
  void *__private_tm[4];
  /* GCC split stack support.  */
  void *__private_ss;
} tcbhead_t;

But the offset of __private_ss for i386 was mistakenly set to 0x30,
instead of 0x34 and libgcc/config/i386/morestack.S has:

	cmpl	%gs:0x30,%eax		# See if we have enough space.
	movl	%eax,%gs:0x30		# Save the new stack boundary.
	movl	%eax,%gs:0x30		# Save the new stack boundary.
	movl	%ecx,%gs:0x30		# Save new stack boundary.
	movl	%eax,%gs:0x30
	movl	%gs:0x30,%eax
	movl	%eax,%gs:0x30

Since update TARGET_THREAD_SPLIT_STACK_OFFSET changes split stack ABI,
glibc 2.28 has been changed by

commit 0221ce2a90be2d40fc90f0b5dcec77a1ec013f53
Author: H.J. Lu <hjl.tools@gmail.com>
Date:   Tue Jun 12 06:23:28 2018 -0700

    i386: Change offset of __private_ss to 0x30 [BZ #23250]

to match GCC:

typedef struct
{
  void *tcb;		/* Pointer to the TCB.  Not necessarily the
			   thread descriptor used by libpthread.  */
  dtv_t *dtv;
  void *self;		/* Pointer to the thread descriptor.  */
  int multiple_threads;
  uintptr_t sysinfo;
  uintptr_t stack_guard;
  uintptr_t pointer_guard;
  int gscope_flag;
  int __glibc_reserved1;
  /* Reservation of some values for the TM ABI.  */
  void *__private_tm[3];
  /* GCC split stack support.  */
  void *__private_ss;
  void *__glibc_reserved2;
} tcbhead_t;

	PR target/85990
	* config/i386/gnu-user.h (TARGET_THREAD_SPLIT_STACK_OFFSET):
	Update comments.
	* config/i386/gnu-user64.h (TARGET_THREAD_SPLIT_STACK_OFFSET):
	Likewise.

git-svn-id: svn+ssh://gcc.gnu.org/svn/gcc/trunk@261501 138bc75d-0d04-0410-961f-82ee72b054a4