[gdb/testsuite] Fix another regexp in gdb.threads/stepi-over-clone.exp

[gdb/testsuite] Fix another regexp in gdb.threads/stepi-over-clone.exp

On openSUSE Tumbleweed, I run into:
...
(gdb) PASS: gdb.threads/stepi-over-clone.exp: catch process syscalls
continue^M
Continuing.^M
^M
Catchpoint 2 (call to syscall clone3), __clone3 () at clone3.S:62^M
(gdb) FAIL: gdb.threads/stepi-over-clone.exp: continue
...

Fix this by updating another (see commit 8fbf220321d) regexp to also recognize
__clone3.

Tested on x86_64-linux.