[libc] Add SIGPOLL si_code values. (#212873) This is a follow-up to efc78d2029b9f716de4546ed88571616d91e826c, which adds `si_code` values for the `SIGPOLL` signal on Linux. GitOrigin-RevId: 6e4374e9749c23028009e8d06870a6c4184d9891