linux: futex v1 API cleanup

* Use `packed struct` for flags arguments.  So, instead of
  `linux.FUTEX.WAIT` use `.{ .cmd = .WAIT, .private = true }`

* rename `futex_wait` and `futex_wake` which didn't actually specify
  wait/wake, as `futex_3arg` and `futex_4arg` (as its the number
  of parameters that is different, the `op` is whatever is specified.

* expose the full six-arg flavor of the syscall (for some of the advanced
  ops), and add packed structs for their arguments.

* Use a `packed union` to support the 4th parameter which is sometimes a
  `timespec` pointer, and sometimes a `u32`.

* Add tests that make sure the structure layout is correct and that the
  basic argument passing is working (no actual futexes are contended).

lib/std/Thread/Futex.zig

@@ -262,10 +262,10 @@ const LinuxImpl = struct {
             ts.nsec = @as(@TypeOf(ts.nsec), @intCast(timeout_ns % std.time.ns_per_s));
         }
 
-        const rc = linux.futex_wait(
-            @as(*const i32, @ptrCast(&ptr.raw)),
-            linux.FUTEX.PRIVATE_FLAG | linux.FUTEX.WAIT,
-            @as(i32, @bitCast(expect)),
+        const rc = linux.futex_4arg(
+            &ptr.raw,
+            .{ .cmd = .WAIT, .private = true },
+            expect,
             if (timeout != null) &ts else null,
         );
 
@@ -284,10 +284,10 @@ const LinuxImpl = struct {
     }
 
     fn wake(ptr: *const atomic.Value(u32), max_waiters: u32) void {
-        const rc = linux.futex_wake(
-            @as(*const i32, @ptrCast(&ptr.raw)),
-            linux.FUTEX.PRIVATE_FLAG | linux.FUTEX.WAKE,
-            std.math.cast(i32, max_waiters) orelse std.math.maxInt(i32),
+        const rc = linux.futex_3arg(
+            &ptr.raw,
+            .{ .cmd = .WAKE, .private = true },
+            @min(max_waiters, std.math.maxInt(i32)),
         );
 
         switch (linux.E.init(rc)) {