Fix s390 GNU/Linux build after enum __ptrace_request changes

Fix s390 GNU/Linux build after enum __ptrace_request changes

The buildbot noticed that the enum __ptrace_request series broke the
s390 GNU/Linux build:

../../binutils-gdb/gdb/s390-linux-nat.c: In function 'fetch_regs':
../../binutils-gdb/gdb/s390-linux-nat.c:226:54: error: macro "ptrace" requires 4 arguments, but only 3 given
   if (ptrace (PTRACE_PEEKUSR_AREA, tid, (long) &parea) < 0)
                                                      ^
../../binutils-gdb/gdb/s390-linux-nat.c: In function 'store_regs':
../../binutils-gdb/gdb/s390-linux-nat.c:243:54: error: macro "ptrace" requires 4 arguments, but only 3 given
   if (ptrace (PTRACE_PEEKUSR_AREA, tid, (long) &parea) < 0)
                                                      ^

Fix this the same way it's handled everywhere else -- just pass 0 as
forth argument, which also handles non-varargs ptrace prototypes in
non-glibc libcs, e.g., Bionic (if it ever gets a s390 port...).

gdb/ChangeLog:
2015-07-24  Pedro Alves  <palves@redhat.com>

	* s390-linux-nat.c (fetch_regs, store_regs, fetch_fpregs)
	(s390_stopped_by_watchpoint, s390_prepare_to_resume): Pass 0 as
	forth argument to ptrace PTRACE_PEEKUSR_AREA/PTRACE_POKEUSR_AREA.