[gdb/testsuite] Fix gdb.server/server-connect.exp for missing ipv6

On a system without ipv6 support enabled, when running test-case
gdb.server/server-connect.exp, it takes about 4 minutes, and I get:
...
builtin_spawn gdbserver --once ::1:2347 server-connect^M
Can't open socket: Address family not supported by protocol.^M
Exiting^M
PASS: gdb.server/server-connect.exp: tcp6: start gdbserver
target remote tcp6:::1:2347^M
A program is being debugged already.  Kill it? (y or n) y^M
could not connect: Address family not supported by protocol.^M
(gdb) FAIL: gdb.server/server-connect.exp: tcp6: connect to gdbserver using tcp6:::1
...

Fix this by:
- recognizing the error message in gdbserver_start, and returning an empty list
  to signal unsupported, and
- handling the unsupported response in the test-case.

This brings testing time down to 2 seconds, and gets me:
...
UNSUPPORTED: gdb.server/server-connect.exp: tcp6: start gdbserver
UNSUPPORTED: gdb.server/server-connect.exp: tcp6-with-brackets: start gdbserver
UNSUPPORTED: gdb.server/server-connect.exp: udp6: start gdbserver
UNSUPPORTED: gdb.server/server-connect.exp: udp6-with-brackets: start gdbserver
...

Tested on aarch64-linux.

Approved-By: Tom Tromey <tom@tromey.com>

PR testsuite/31502
Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=31502