Add Linux PTRACE_EVENT_STOP

Add PTRACE_EVENT_STOP value to Linux's sys/ptrace.h, modify related
comments accordingly.

This constant initially appeared in Linux 3.1 (kernel commit 3544d72a,
"ptrace: implement PTRACE_SEIZE") but its value has changed later
in Linux 3.4 (kernel commit 5cdf389a, "ptrace: renumber
PTRACE_EVENT_STOP so that future new options and events can match").

The comment is also taken from the above commit.

This constant is used by e.g. strace, CRIU, Mozilla RR.

* sysdeps/unix/sysv/linux/aarch64/sys/ptrace.h (__ptrace_eventcodes):
Add PTRACE_EVENT_STOP.
* sysdeps/unix/sysv/linux/ia64/sys/ptrace.h: Likewise.
* sysdeps/unix/sysv/linux/powerpc/sys/ptrace.h: Likewise.
* sysdeps/unix/sysv/linux/s390/sys/ptrace.h: Likewise.
* sysdeps/unix/sysv/linux/sparc/sys/ptrace.h: Likewise.
* sysdeps/unix/sysv/linux/sys/ptrace.h: Likewise.
* sysdeps/unix/sysv/linux/tile/sys/ptrace.h: Likewise.