This matches all other platforms. Even if this field is defined as 'int'
in the C definition, the expectation is that the full 32-bit unsigned
integer range can be used. In particular this Sigaction initializer in
the new std.debug code was causing a build failure:

```zig
.flags = (posix.SA.SIGINFO | posix.SA.RESTART | posix.SA.RESETHAND)
```