[libc] Add SCM_TIMESTAMP constants (#232688)

These are the control message types for SO_TIMESTAMP, SO_TIMESTAMPNS,
and SO_TIMESTAMPING. On Linux, they are defined as aliases for their SO_
counterparts.

Assisted-by: Gemini
GitOrigin-RevId: a561104a8cb8d00019eca7b079526ae81d6c4214