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

On fedora rawhide, I ran into:
...
(gdb) continue^M
Continuing.^M
^M
Catchpoint 2 (call to syscall clone3), 0x000000000042097d in __clone3 ()^M
(gdb) FAIL: gdb.threads/stepi-over-clone.exp: continue
...

Fix this by updating a regexp to also recognize __clone3.

Tested on x86_64-linux.

Tested-By: Guinevere Larsen <blarsen@redhat.com>