libstdc++: Define effective-target for net::ip features [PR 100351]

libstdc++: Define effective-target for net::ip features [PR 100351]

Define a new effective-target keyword so that tests for the Networking
TS <experimental/internet> header can be skipped on targets where none
of it can be usefully defined.

libstdc++-v3/ChangeLog:

	PR libstdc++/100180
	PR libstdc++/100286
	PR libstdc++/100351
	* testsuite/experimental/net/internet/address/v4/comparisons.cc:
	Use new effective-target keyword.
	* testsuite/experimental/net/internet/address/v4/cons.cc:
	Likewise.
	* testsuite/experimental/net/internet/address/v4/creation.cc:
	Likewise.
	* testsuite/experimental/net/internet/address/v4/members.cc:
	Likewise.
	* testsuite/experimental/net/internet/address/v6/members.cc:
	Likewise.
	* testsuite/experimental/net/internet/resolver/base.cc:
	Likewise.
	* testsuite/experimental/net/internet/resolver/ops/lookup.cc:
	Likewise.
	* testsuite/experimental/net/internet/resolver/ops/reverse.cc:
	Likewise.
	* testsuite/experimental/net/internet/socket/opt.cc:
	Likewise.
	* testsuite/experimental/net/internet/tcp.cc:
	Likewise.
	* testsuite/experimental/net/internet/udp.cc:
	Likewise.
	* testsuite/lib/libstdc++.exp (check_effective_target_net_ts_ip):
	New proc to check net_ts_ip et.