Fix At flags for real

The fix got reverted in the merge conflict but now properly fix `At`

Update posix `At` to use packed struct (I was holding back from touching
`posix` and `c` but I might as well just get this done instead of using
the deprecated constants since the diff is already big and At is worth
while)

Signed-off-by: Bernard Assan <mega.alpha100@gmail.com>

Author: bernardassan

lib/std/Io/Dir.zig

@@ -24,7 +24,7 @@ pub fn cwd() Dir {
     return switch (native_os) {
         .windows => .{ .handle = std.os.windows.peb().ProcessParameters.CurrentDirectory.Handle },
         .wasi => .{ .handle = std.options.wasiCwd() },
-        else => .{ .handle = std.posix.AT.FDCWD },
+        else => .{ .handle = std.posix.At.fdcwd },
     };
 }
 

lib/std/Io/Threaded.zig

@@ -1403,12 +1403,12 @@ fn dirStatPathPosix(
     var path_buffer: [posix.PATH_MAX]u8 = undefined;
     const sub_path_posix = try pathToPosix(sub_path, &path_buffer);
 
-    const flags: u32 = if (!options.follow_symlinks) posix.AT.SYMLINK_NOFOLLOW else 0;
+    const flags: posix.At = .{ .symlink_nofollow = if (!options.follow_symlinks) true else false };
 
     while (true) {
         try t.checkCancel();
         var stat = std.mem.zeroes(posix.Stat);
-        switch (posix.errno(fstatat_sym(dir.handle, sub_path_posix, &stat, flags))) {
+        switch (posix.errno(fstatat_sym(dir.handle, sub_path_posix, &stat, @bitCast(flags)))) {
             .SUCCESS => return statFromPosix(&stat),
             .INTR => continue,
             .CANCELED => return error.Canceled,
@@ -1632,7 +1632,7 @@ fn dirAccessPosix(
     var path_buffer: [posix.PATH_MAX]u8 = undefined;
     const sub_path_posix = try pathToPosix(sub_path, &path_buffer);
 
-    const flags: u32 = @as(u32, if (!options.follow_symlinks) posix.AT.SYMLINK_NOFOLLOW else 0);
+    const flags: posix.At = .{ .symlink_nofollow = if (!options.follow_symlinks) true else false };
 
     const mode: u32 =
         @as(u32, if (options.read) posix.R_OK else 0) |
@@ -1641,7 +1641,7 @@ fn dirAccessPosix(
 
     while (true) {
         try t.checkCancel();
-        switch (posix.errno(posix.system.faccessat(dir.handle, sub_path_posix, mode, flags))) {
+        switch (posix.errno(posix.system.faccessat(dir.handle, sub_path_posix, mode, @bitCast(flags)))) {
             .SUCCESS => return,
             .INTR => continue,
             .CANCELED => return error.Canceled,
@@ -2901,7 +2901,7 @@ fn fileSeekTo(userdata: ?*anyopaque, file: Io.File, offset: u64) Io.File.SeekErr
 fn openSelfExe(userdata: ?*anyopaque, flags: Io.File.OpenFlags) Io.File.OpenSelfExeError!Io.File {
     const t: *Threaded = @ptrCast(@alignCast(userdata));
     switch (native_os) {
-        .linux, .serenity => return dirOpenFilePosix(t, .{ .handle = posix.AT.FDCWD }, "/proc/self/exe", flags),
+        .linux, .serenity => return dirOpenFilePosix(t, .{ .handle = posix.At.fdcwd }, "/proc/self/exe", flags),
         .windows => {
             // If ImagePathName is a symlink, then it will contain the path of the symlink,
             // not the path that the symlink points to. However, because we are opening