Implement W as packed struct in c.zig

fixes the below on CI

/lib/std/process/Child.zig:526:24: error: cannot @bitcast to 'c.W__struct_316926'; struct does not have a guaranteed in-memory layout
    const w: posix.W = @bitcast(status);
                       ^~~~~~~~~~~~~~~~
/lib/std/c.zig:3682:75: note: struct declared here
    .driverkit, .ios, .maccatalyst, .macos, .tvos, .visionos, .watchos => struct {
                                                                          ^~~~~~

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

Author: bernardassan