runtime: use WCLONE when waiting on pidfd test child
As of CL 650835, the pidfd test child no longer sends SIGCHLD on exit.
Per clone(2), "If [the child termination] signal is specified as
anything other than SIGCHLD, then the parent process must specify the
__WALL or __WCLONE options when waiting for the child with wait(2)."
Align with this requirement.
For #71828.
Change-Id: I6a6a636c739e4a59abe1533fe429a433e8588939
Reviewed-on: https://go-review.googlesource.com/c/go/+/651415 Reviewed-by: Ian Lance Taylor <iant@golang.org>
Auto-Submit: Michael Pratt <mpratt@google.com> Reviewed-by: Ian Lance Taylor <iant@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>