[gdb/tdep] Fix s390_linux_nat_target::stopped_by_watchpoint

[gdb/tdep] Fix s390_linux_nat_target::stopped_by_watchpoint

On s390x-linux, I run into:
...
(gdb) continue^M
Continuing.^M
breakpoint.c:5784: internal-error: bpstat_stop_status_nowatch: \
  Assertion `!target_stopped_by_watchpoint ()' failed.^M
A problem internal to GDB has been detected,^M
further debugging may prove unreliable.^M
FAIL: gdb.threads/watchpoint-fork.exp: parent: singlethreaded: \
  breakpoint after the first fork (GDB internal error)
...

What happens is the follow:
- a watchpoint event triggers
- the event is processed, s390_linux_nat_target::stopped_by_watchpoint is called and
  it returns true, as expected
- the watchpoint event is reported by gdb, and gdb stops
- we issue a continue command
- a fork event triggers
- the event is processed, and during processing that event
  s390_linux_nat_target::stopped_by_watchpoint is called again, and returns
  true
- the assertion fails, because the function is expected to return false

The function s390_linux_nat_target::stopped_by_watchpoint returns true the
second time, because it looks at the exact same data that was looked at when
it was called the first time, and that data hasn't changed.

There's code in the same function that intends to prevent that from happening:
...
      /* Do not report this watchpoint again.  */
      memset (&per_lowcore, 0, sizeof (per_lowcore));
      if (ptrace (PTRACE_POKEUSR_AREA, s390_inferior_tid (), &parea, 0) < 0)
       perror_with_name (_("Couldn't clear watchpoint status"));
...
and that probably used to work for older kernels, but no longer does since
linux kernel commit 5e9a26928f55 ("[S390] ptrace cleanup").

Fix this by copying this:
...
  siginfo_t siginfo;
  if (!linux_nat_get_siginfo (inferior_ptid, &siginfo))
    return false;
  if (siginfo.si_signo != SIGTRAP
      || (siginfo.si_code & 0xffff) != TRAP_HWBKPT)
    return false;
...
from aarch64_linux_nat_target::stopped_data_address and remove the
obsolete watchpoint status clearing code.

Tested on s390x-linux.

Approved-By: Ulrich Weigand <uweigand@de.ibm.com>