[release-branch.go1.23] syscall: don't send child signal when testing pidfd

[release-branch.go1.23] syscall: don't send child signal when testing pidfd

Avoid a spurious SIGCHLD the first time we start a process.

For #71828
Fixes #71848

Change-Id: I744100d21bf6aaaaafc99bc5eec9f9f807a50682
Reviewed-on: https://go-review.googlesource.com/c/go/+/651055
Reviewed-by: Michael Knyszek <mknyszek@google.com>
Reviewed-by: Michael Pratt <mpratt@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>