[gdb/tdep] Fix gdb.base/siginfo.exp on s390x-linux

On s390x-linux (SLES 15 SP5), I'm running into:
...
FAIL: gdb.base/siginfo.exp: backtrace for nexti (pattern 2)
FAIL: gdb.base/siginfo.exp: step out of handler
...

The first FAIL is caused by a failure to unwind:
...
 (gdb) bt^M
 #0  handler (sig=26, info=0x3ffffffe428, context=0x3ffffffe4a8) at \
   gdb.base/siginfo.c:31^M
 Backtrace stopped: Cannot access memory at address 0x1a00000088^M
 (gdb)
...

In contrast, on x86_64-linux I get instead:
...
 (gdb) bt^M
 #0  handler (sig=26, info=0x7fffffffc170, context=0x7fffffffc040) at \
   gdb.base/siginfo.c:31^M
 #1  <signal handler called>^M
 #2  0x0000000000401201 in main () at gdb.base/siginfo.c:67^M
 (gdb)
...

The memory access error is triggered here in s390_sigtramp_frame_unwind_cache:
...
  /* Restore the previous frame's SP.  */
  prev_sp = read_memory_unsigned_integer (
			info->saved_regs[S390_SP_REGNUM].addr (),
                        word_size, byte_order);
...
while trying to read an "Old-style RT frame" (for syscall sigreturn).

The problem is that we actually have a "New-style RT frame" (for syscall
rt_sigreturn).

[ See linux kernel source file arch/s390/kernel/signal.c for a detailed
explanation of the two. ]

The choice between the two is made earlier in that same function:
...
  /* New-style RT frame:
	retcode + alignment (8 bytes)
	siginfo (128 bytes)
	ucontext (contains sigregs at offset 5 words).  */
  if (next_ra == next_cfa)
    {
      ...
    }

  /* Old-style RT frame and all non-RT frames:
	old signal mask (8 bytes)
	pointer to sigregs.  */
  else
...

I'm not sure why the check gives the wrong result, but I noticed that
s390_sigtramp_frame_sniffer is able to distinguish between the two, so fix
this by:
- factoring out new function s390_sigtramp_p out of
  s390_sigtramp_frame_sniffer, and
- using s390_sigtramp_p in s390_sigtramp_frame_unwind_cache to distinguish
  between the "Old-style RT frame" and "New-style RT frame".

This fixes the backtrace.

The second failure is:
...
(gdb) step^M
32      } /* handler */^M
1: x/i $pc^M
=> 0x1000772 <handler+50>:      nopr^M
(gdb) step^M
0x000003fffdffe490 in __kernel_rt_sigreturn ()^M
1: x/i $pc^M
=> 0x3fffdffe490 <__kernel_rt_sigreturn>:       svc     173^M
(gdb) FAIL: gdb.base/siginfo.exp: step out of handler
...

There is some code in process_event_stop_test that is supposed to trigger:
...
  if (ecs->event_thread->control.step_range_end != 1
      && (ecs->event_thread->control.step_over_calls == STEP_OVER_UNDEBUGGABLE
	  || ecs->event_thread->control.step_over_calls == STEP_OVER_ALL)
      && get_frame_type (frame) == SIGTRAMP_FRAME)
    {
      infrun_debug_printf ("stepped into signal trampoline");
      /* The inferior, while doing a "step" or "next", has ended up in
	 a signal trampoline (either by a signal being delivered or by
	 the signal handler returning).  Just single-step until the
	 inferior leaves the trampoline (either by calling the handler
	 or returning).  */
      keep_going (ecs);
      return;
    }
...
but it doesn't because frame is a NORMAL_FRAME instead of a SIGTRAMP_FRAME.

This is caused by the "dwarf2" unwinder triggering, which has higher priority
than the "s390 linux sigtramp" unwinder:
...
(gdb) maint info frame-unwinders
Name                        Type                      Class     Enabled
dummy                       DUMMY_FRAME               GDB       Y
dwarf2 tailcall             TAILCALL_FRAME            DEBUGINFO Y
inline                      INLINE_FRAME              GDB       Y
jit                         NORMAL_FRAME              EXTENSION Y
python                      NORMAL_FRAME              EXTENSION Y
dwarf2                      NORMAL_FRAME              DEBUGINFO Y
dwarf2 signal               SIGTRAMP_FRAME            DEBUGINFO Y
s390 linux sigtramp         SIGTRAMP_FRAME            ARCH      Y
s390 stub                   NORMAL_FRAME              ARCH      Y
s390 prologue               NORMAL_FRAME              ARCH      Y
...

I found some code in dwarf2_frame_sniffer:
...
  /* On some targets, signal trampolines may have unwind information.
     We need to recognize them so that we set the frame type
     correctly.  */

  if (fde->cie->signal_frame
      || dwarf2_frame_signal_frame_p (get_frame_arch (this_frame),
				      this_frame))
    return self->type () == SIGTRAMP_FRAME;
...
and an example implementation i386_linux_dwarf_signal_frame_p, and after
copying this approach, indeed the stepping failure was fixed, but the
backtrace broken again.

Instead, fix this by giving the "s390 linux sigtramp" unwinder a higher
priority:
...
(gdb) maint info frame-unwinders
Name                        Type                      Class     Enabled
dummy                       DUMMY_FRAME               GDB       Y
dwarf2 tailcall             TAILCALL_FRAME            DEBUGINFO Y
inline                      INLINE_FRAME              GDB       Y
jit                         NORMAL_FRAME              EXTENSION Y
python                      NORMAL_FRAME              EXTENSION Y
s390 linux sigtramp         SIGTRAMP_FRAME            ARCH      Y
dwarf2                      NORMAL_FRAME              DEBUGINFO Y
dwarf2 signal               SIGTRAMP_FRAME            DEBUGINFO Y
s390 stub                   NORMAL_FRAME              ARCH      Y
s390 prologue               NORMAL_FRAME              ARCH      Y
...

Also fixes test-case gdb.base/sigaltstack.exp and gdb.base/sigbpt.exp.

Tested on s390x-linux.

Reviewed-By: Keith Seitz <keiths@redhat.com>

PR tdep/33708
Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=33708