Handle additional connection error

Handle additional connection error

On Ubuntu 18.04/20.04 I was running into annoying timeouts for
gdb.server/server-connect.exp. Those were caused by the ipv6 tests, because
they were running into the "Cannot assign requested address" error, originated
from the connect syscall.

Improve this by handling this additional error in the testsuite library.

It still fails for me, but at least it fails pretty quickly and doesn't make the
testsuite run take longer.

gdb/testsuite/ChangeLog:

2021-01-21  Luis Machado  <luis.machado@linaro.org>

	* lib/gdbserver-support.exp (gdb_target_cmd_ext): Handle a new error
	message.