S390: fix sys/ptrace.h to make it includible again after asm/ptrace.h

sys/ptrace.h on S390 used to be includible both before and after
asm/ptrace.h, until commit b08a6a0d
among other changes introduced PTRACE_SINGLEBLOCK enum constant which
is also defined in asm/ptrace.h as a macro, making sys/ptrace.h fail
to compile when included after asm/ptrace.h.

* sysdeps/unix/sysv/linux/s390/sys/ptrace.h [_LINUX_PTRACE_H ||
_S390_PTRACE_H]: Undefine all PTRACE_* macro constants defined
later as enum constants, except PTRACE_PEEKUSER, PTRACE_POKEUSER,
and PTRACE_SEIZE_DEVEL that are not defined by Linux headers.