[gdb/testsuite] Fix gdb.threads/multiple-successive-infcall.exp on native-gdbserver

With test-case gdb.threads/multiple-successive-infcall.exp and target board
native-gdbserver I run into:
...
(gdb) continue^M
Continuing.^M
[New Thread 758.759]^M
^M
Thread 1 "multiple-succes" hit Breakpoint 2, main () at \
  multiple-successive-infcall.c:97^M
97            thread_ids[tid] = tid + 2; /* prethreadcreationmarker */^M
(gdb) FAIL: gdb.threads/multiple-successive-infcall.exp: thread=5: \
  created new thread
...

The problem is that the new thread message doesn't match the regexp, which
expects something like this instead:
...
[New Thread 0x7ffff746e700 (LWP 570)]^M
...

Fix this by accepting this form of new thread message.

Tested on x86_64-linux.