[gdb/testsuite] Fix gdb.server/server-kill.exp with -m32

[gdb/testsuite] Fix gdb.server/server-kill.exp with -m32

When running test-case gdb.server/server-kill.exp with target board unix/-m32,
I run into:
...
0xf7fd6b20 in _start () from /lib/ld-linux.so.2^M
(gdb) Executing on target: kill -9 13082    (timeout = 300)
builtin_spawn -ignore SIGHUP kill -9 13082^M
bt^M
(gdb) FAIL: gdb.server/server-kill.exp: kill_pid_of=server: test_unwind_syms: bt
...

The test-case expects the backtrace command to trigger remote communication,
which then should result in a "Remote connection closed" or similar.

However, no remote communication is triggered, because we hit the "Check that
this frame is unwindable" case in get_prev_frame_always_1.

We don't hit this problem in the kill_pid_of=inferior case, because there we
run to main before doing the backtrace.

Fix this by doing the same in the kill_pid_of=server case.

Tested on x86_64-linux.