[gdb/testsuite] Fix gdb.threads/killed-outside.exp with -m32

When running test-case gdb.threads/killed-outside.exp with target board
unix/-m32, we run into:
...
(gdb) PASS: gdb.threads/killed-outside.exp: get pid of inferior
Executing on target: kill -9 10969    (timeout = 300)
spawn -ignore SIGHUP kill -9 10969^M
continue^M
Continuing.^M
[Thread 0xf7cb4b40 (LWP 10973) exited]^M
^M
Program terminated with signal SIGKILL, Killed.^M
The program no longer exists.^M
(gdb) FAIL: gdb.threads/killed-outside.exp: prompt after first continue
...

Fix this by allowing this output.

Tested on x86_64-linux.

gdb/testsuite/ChangeLog:

2021-01-26  Tom de Vries  <tdevries@suse.de>

	* gdb.threads/killed-outside.exp: Allow regular output.