[release-branch.go1.24] 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 #71849.
For #71828.

Change-Id: I6a6a636c739e4a59abe1533fe429a433e8588939
Reviewed-on: https://go-review.googlesource.com/c/go/+/651415
Reviewed-by: Ian Lance Taylor <[email protected]>
Auto-Submit: Michael Pratt <[email protected]>
Reviewed-by: Ian Lance Taylor <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>
(cherry picked from commit e1e65ae)
Reviewed-on: https://go-review.googlesource.com/c/go/+/651476

Author: prattmic

src/syscall/exec_linux.go

@@ -779,7 +779,12 @@ func os_checkClonePidfd() error {
 		var err error
 		for {
 			var status WaitStatus
-			_, err = Wait4(int(pid), &status, 0, nil)
+			// WCLONE is an untyped constant that sets bit 31, so
+			// it cannot convert directly to int on 32-bit
+			// GOARCHes. We must convert through another type
+			// first.
+			flags := uint(WCLONE)
+			_, err = Wait4(int(pid), &status, int(flags), nil)
 			if err != EINTR {
 				break
 			}
@@ -797,7 +802,7 @@ func os_checkClonePidfd() error {
 
 	for {
 		const _P_PIDFD = 3
-		_, _, errno = Syscall6(SYS_WAITID, _P_PIDFD, uintptr(pidfd), 0, WEXITED, 0, 0)
+		_, _, errno = Syscall6(SYS_WAITID, _P_PIDFD, uintptr(pidfd), 0, WEXITED | WCLONE, 0, 0)
 		if errno != EINTR {
 			break
 		}