[gdb/testsuite] Fix FAIL in gdb.base/annota1.exp

On openSUSE tumbleweed I run into:
...
FAIL: gdb.base/annota1.exp: run until main breakpoint (timeout)
...
due to a message related to libthread_db:
...
^Z^Zstarting^M
[Thread debugging using libthread_db enabled]^M
Using host libthread_db library "/lib64/libthread_db.so.1".^M
^M
^Z^Zframes-invalid^M
...
which is not matched by the regexp.

Fix this by updating the regexp.

Tested on x86_64-linux.