Change SIGCHLD to SIGKILL in ptrace stubs.
Change SIGCHLD to SIGKILL in ptrace stubs.

If the child stubs are killed by any unmaskable signal (e.g. SIGKILL), then
the parent process will similarly be killed, resulting in the death of all
other stubs.

The effect of this is that if the OOM killer selects and kills a stub, the
effect is the same as though the OOM killer selected and killed the sentry.

PiperOrigin-RevId: 202219984
Change-Id: I0b638ce7e59e0a0f4d5cde12a7d05242673049d7

google/gvisor,google/gvisor,google/gvisor,google/gvisor,google/gvisor,google/gvisor,google/gvisor,google/gvisor