[gdb/testsuite] Use ssh -t in remote-*.exp

[gdb/testsuite] Use ssh -t in remote-*.exp

When running test-case gdb.server/multi-ui-errors.exp on target board
remote-gdbserver-on-localhost.exp, I run into:
...
(gdb) PASS: gdb.server/multi-ui-errors.exp: connect to gdbserver
continue^M
Continuing.^M
PASS: gdb.server/multi-ui-errors.exp: continue - extra UI
Remote debugging from host ::1, port 35466^M
FAIL: gdb.server/multi-ui-errors.exp: ensure inferior is running
...

The problem is that the target board uses ssh -T, which fails to guarantee
that output from the inferior will be available.

Fix this by copying proc ${board}_spawn from local-remote-host.exp, which
ensures using ssh -t.  [ It would be nice to define an ssh base board to
get rid of the copies, but I'm not addressing that in this commit. ]

Likewise for target board remote-stdio-gdbserver.exp.

Tested on x86_64-linux.