libstdc++: xfail experimental/net tests on rtems

libstdc++: xfail experimental/net tests on rtems

Some net/timer/waitable tests fail on rtems because poll() is not
available.

The above, as well as net/internet/resolver/ops tests and
net/timer/waitable/cons.cc, will fail early at runtime unless mkfifo
is enabled in the RTEMS configuration, because the io_context ctor
throws when pipe() fails.

However, even enabling pipes and adjusting the net_ts link command to
use --gc-sections for -lbsd as recommended, both
net/internet/resolver/ops still fail at runtime.

for  libstdc++-v3/ChangeLog

	* testsuite/lib/dg-options.exp (add_options_for_net_ts):
	Add -Wl,--gc-sections for RTEMS targets.
	* testsuite/experimental/net/timer/waitable/dest.cc: Link-time
	xfail on RTEMS.
	* testsuite/experimental/net/timer/waitable/ops.cc: Likewise.
	* testsuite/experimental/net/internet/resolver/ops/lookup.cc:
	Execution-time xfail on RTEMS.
	* testsuite/experimental/net/internet/resolver/ops/reverse.cc:
	Likewise.