[gdb/testsuite] Fix local-remote-host-native.exp for gdb.server tests

[gdb/testsuite] Fix local-remote-host-native.exp for gdb.server tests

When running test-case gdb.server/stop-reply-no-thread-multi.exp with
host+target board local-remote-host-native, I run into a time-out:
...
(gdb) PASS: gdb.server/stop-reply-no-thread-multi.exp: target-non-stop=off: \
  to_disable=: disconnect
builtin_spawn /usr/bin/ssh -t -l vries 127.0.0.1 gdbserver --once \
  localhost:2346 stop-reply-no-thread-multi^M
Process stop-reply-no-thread-multi created; pid = 32600^M
Listening on port 2346^M
set remote threads-packet off^M
FAIL: gdb.server/stop-reply-no-thread-multi.exp: target-non-stop=off: \
  to_disable=: set remote threads-packet off (timeout)
...

This is due to this line in ${board}_spawn:
...
    set board_info($board,fileid) $spawn_id
...

We have the following series of events:
- gdb is spawned, setting fileid
- a few gdb commands (set height etc) are send using fileid, arrive at gdb and
  are successful
- gdbserver is spawned, overwriting fileid
- the next gdb command is sent using fileid, so it's send
  to gdbserver instead of gdb, and we run into the timeout.

There is some notion of current gdb, tracked in both gdb_spawn_id and fileid
of the host board (see switch_gdb_spawn_id).  And because the host and target
board are the same, spawning something on the target overwrites the fileid on
host, and consequently the current gdb.

Fix this by only setting fileid when spawning gdb.

Tested on x86_64-linux.

Now gdb.server/*.exp passes for host+target board local-remote-host-native,
except for file-transfer.exp.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=29734