os: spurious SIGCHILD on running child process [1.24 backport]

[gopherbot]

@ianlancetaylor requested issue #71828 to be considered for backport to the next 1.24 minor release.

@gopherbot Please open backport issues.

This bug causes a spurious SIGCHLD signal the first time a process is executed when running on Linux systems. This is a regression from past behavior and from behavior on non-Linux systems. The patch is small and safe.

[gopherbot]

Change https://go.dev/cl/651035 mentions this issue: [release-branch.go1.24] syscall: don't send child signal when testing pidfd

[gopherbot]

Change https://go.dev/cl/651476 mentions this issue: [release-branch.go1.24] runtime: use WCLONE when waiting on pidfd test child

[prattmic]

https://go.dev/cl/651035 contains a bug, which is fixed by https://go.dev/cl/651035, so both must be cherry-picked together.

[gopherbot]

Closed by merging CL 651476 (commit becc17e) to release-branch.go1.24.

[gopherbot]

Closed by merging CL 651035 (commit d418e22) to release-branch.go1.24.