std.Io.Threaded: glibc implementation of `netLookup`

[andrewrk]

Extracted from #25592.

The glibc implementation of netLookup should use getaddrinfo_a / gai_cancel to obtain these two properties:

• asynchronously add to results queue
• cancellation integration

 if (builtin.target.isGnuLibC()) {
 // TODO use getaddrinfo_a / gai_cancel
 }