Use ad-hoc struct type for clktck

Also add a TODO comment to switch to sync.OnceValue once Go 1.21 is the
oldest supported version.

sysconf_netbsd.go

@@ -25,10 +25,10 @@ const (
 	_POSIX2_UPE       = -1
 )
 
-var (
-	clktck     int64
-	clktckOnce sync.Once
-)
+var clktck struct {
+	sync.Once
+	v int64
+}
 
 func sysconfPOSIX(name int) (int64, error) {
 	// NetBSD does not define all _POSIX_* values used in sysconf_posix.go
@@ -42,7 +42,6 @@ func sysconf(name int) (int64, error) {
 	// Duplicate the relevant values here.
 
 	switch name {
-
 	// 1003.1
 	case SC_ARG_MAX:
 		return sysctl32("kern.argmax"), nil
@@ -55,13 +54,14 @@ func sysconf(name int) (int64, error) {
 		}
 		return -1, nil
 	case SC_CLK_TCK:
-		clktckOnce.Do(func() {
-			clktck = -1
+		// TODO: use sync.OnceValue once Go 1.21 is the minimal supported version
+		clktck.Do(func() {
+			clktck.v = -1
 			if ci, err := unix.SysctlClockinfo("kern.clockrate"); err == nil {
-				clktck = int64(ci.Hz)
+				clktck.v = int64(ci.Hz)
 			}
 		})
-		return clktck, nil
+		return clktck.v, nil
 	case SC_NGROUPS_MAX:
 		return sysctl32("kern.ngroups"), nil
 	case SC_JOB_CONTROL: