Adjust gdb.base/memops-watchpoint.exp for Cygwin/Windows

Adjust gdb.base/memops-watchpoint.exp for Cygwin/Windows

When testing gdb.base/memops-watchpoint.exp on Cygwin, I see:

 ...
 continue
 Continuing.

 Thread 1 "memops-watchpoint" hit Hardware watchpoint 2: -location a[28]

 Old value = 104 'h'
 New value = 0 '\000'
 memset () at /usr/src/debug/cygwin-3.5.4-1/winsup/cygwin/x86_64/memset.S:68
 68              rep     stosq
 (gdb) FAIL: gdb.base/memops-watchpoint.exp: continue until memset watchpoint hits

Etc.

This fails on Windows because there all inferiors are multi-threaded
(because both Windows and the Cygwin runtime spawn helper threads), so
we always see the

  'Thread N "foo" hit'

prefix, which the testcase isn't expecting.

Fix it by optionally expecting such the "Thread ... hit" prefix bit.

Change-Id: Ia06eaf1e152f512f39324d04298e6784eca3e537