Fix gdb.trace/mi-traceframe-changed.exp on s390.

Fix gdb.trace/mi-traceframe-changed.exp on s390.

The test fails on s390 with:

  -trace-find frame-number 0^M
  &"PC not available\n"^M
  ^done,found="1",tracepoint="1",traceframe="0",frame={level="-1",addr="<unavailable>",func="??",args=[]}^M
  (gdb) ^M
  FAIL: gdb.trace/mi-traceframe-changed.exp: tfile: -trace-find frame-number 0

tfile knows to infer the PC from the tracepoint's address if the PC
wasn't collected (tfile_fetch_registers) but, that only works on
targets whose PC register is a raw register, and on s390, the PC
register is a pseudo register.

But even if GDB doesn't know how to infer the value of PC, saying the
current frame is level -1 is a bug:

  ^done,found="1",tracepoint="1",traceframe="0",frame={level="-1",addr="<unavailable>",func="??",args=[]}^M
                                                       ^^^^^^^^^

'-1' is the level of the sentinel frame, which should never be visible.

This is caused by the s390's heuristic unwinder accepting the frame
(the fallback heuristic unwinders _always_ accept the frame), but then
the unwind->this_id method throws that "PC not available\n" error.

IOW, the s390's heuristic unwinder was never adjusted to handle
unavailable register values gracefully, which can happen with e.g., a
trimmed core file too.

This is just the minimal necessary for
<unavailable> frames, which at least gets us:

  (gdb) tfind
  Found trace frame 0, tracepoint 1
  #0  <unavailable> in ?? ()

That is, frame #0 instead of -1.

We could get better info out of "info frame" (this patch makes us show
"outermost"), but this change would still be necessary.

gdb/
2014-01-16  Pedro Alves  <palves@redhat.com>

	* s390-linux-tdep.c (s390_frame_unwind_cache): Swallow
	NOT_AVAILABLE_ERROR errors while parsing the prologue or reading
	the backchain.