pkg/fileutil: use fcntl syscall wrappers from golang.org/x/sys/unix (#12316)

Direct syscalls using syscall.Syscall(SYS_*, ...) should no longer be
used on darwin, see [1]. Instead, use the fcntl libSystem wrappers
provided by the golang.org/x/sys/unix package which implement the same
functionality.

[1] https://golang.org/doc/go1.12#darwin

bill-of-materials.json

@@ -369,7 +369,7 @@
 		]
 	},
 	{
-		"project": "golang.org/x/sys/unix",
+		"project": "golang.org/x/sys",
 		"licenses": [
 			{
 				"type": "BSD 3-clause \"New\" or \"Revised\" License",