[libc] Implement sockatmark (#205400)

This patch implements sockatmark as a simple wrapper around
ioctl(SIOCATMARK).

I don't test it with actual out of band data, as that requires a TCP
connection, which is more complicated to set up.

Assisted by Gemini.

GitOrigin-RevId: 753ceecfdcdf43d626ddd9534cdf470572a8deda