[gdb/testsuite] Fix timeouts in gdb.threads/step-over-thread-exit.exp

[gdb/testsuite] Fix timeouts in gdb.threads/step-over-thread-exit.exp

Once in a while, I run into a timeout in test-case
gdb.threads/step-over-thread-exit.exp:
...
(gdb) continue^M
Continuing.^M
[New Thread 0xfffff7cff1a0 (LWP 2874854)]^M
^M
Thread 97 "step-over-threa" hit Breakpoint 2, 0x0000000000410314 in \
  my_exit_syscall () at gdb/testsuite/lib/my-syscalls.S:74^M
74      SYSCALL (my_exit, __NR_exit)^M
(gdb) [Thread 0xfffff7cff1a0 (LWP 2874853) exited]^M
FAIL: $exp: step_over_mode=displaced: non-stop=on: target-non-stop=on: \
  schedlock=off: cmd=continue: ns_stop_all=0: iter 95: continue (timeout)
...

I can reproduce it more frequently by running with taskset -c <slow core id>.

Fix this by using -no-prompt-anchor.

This requires us to add -no-prompt-anchor to proc gdb_test_multiple.

Tested on aarch64-linux.

PR testsuite/32489
Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=32489