Adds the `tcflag_t` type to the termios constants.
This is made to allow bitwise operations on the termios
constants without an integer cast, e.g.:

```zig
var raw = try std.os.tcgetattr(std.os.STDIN_FILENO);
raw.lflag &= std.os.linux.ECHO | std.os.linux.ICANON;
```
instead of

```zig
var raw = try std.os.tcgetattr(std.os.STDIN_FILENO);
raw.lflag &= ~@intCast(u32, std.os.linux.ECHO | std.os.linux.ICANON);
```

Contributes to #10181