[gdb/testsuite] Fix gdb.threads/killed-outside.exp on aarch64

[gdb/testsuite] Fix gdb.threads/killed-outside.exp on aarch64

On aarch64 (and likewise on arm), I run into:
...
(gdb) PASS: gdb.threads/killed-outside.exp: get pid of inferior
Executing on target: kill -9 11516    (timeout = 300)
builtin_spawn -ignore SIGHUP kill -9 11516^M
continue^M
Continuing.^M
Unable to fetch general registers: No such process.^M
(gdb) [Thread 0xfffff7d511e0 (LWP 11518) exited]^M
^M
Program terminated with signal SIGKILL, Killed.^M
The program no longer exists.^M
FAIL: gdb.threads/killed-outside.exp: prompt after first continue (timeout)
...
due to a mismatch between the actual "No such process" line and the expected
one:
...
set no_such_process_msg "Couldn't get registers: No such process\."
...

Fix this by updating the regexp.

Tested on aarch64-linux, and x86_64-linux.