gdb/testsuite: fix gdb.threads/linux-dp.exp regex

gdb/testsuite: fix gdb.threads/linux-dp.exp regex

On ubuntu 22.04 with the libc6-dbg package installed, I have the
following failure:

    where
    #0  print_philosopher (n=3, left=33 '!', right=33 '!') at .../gdb/testsuite/gdb.threads/linux-dp.c:105
    #1  0x000055555555576a in philosopher (data=0x55555555937c) at .../gdb/testsuite/gdb.threads/linux-dp.c:148
    #2  0x00007ffff7e11b43 in start_thread (arg=<optimized out>) at ./nptl/pthread_create.c:442
    #3  0x00007ffff7ea3a00 in clone3 () at ../sysdeps/unix/sysv/linux/x86_64/clone3.S:81
    (gdb) FAIL: gdb.threads/linux-dp.exp: first thread-specific breakpoint hit

The regex for this test accounts for different situations (with /
without debug symbol) but assumes that if debug info is present the
backtrace shows execution under pthread_create.  However, for the
implementation under test, we are under start_thread.

Update the regex to accept start_thread.

Tested on Ubuntu-22.04 x86_64 with and without libc6-dbg debug symbols
available.

Change-Id: I1e1536279890bca2cd07f038e026b41e46af44e0