Fix watchpoints with multiple threads on Windows

Fix watchpoints with multiple threads on Windows

A recent internal change pointed out that watchpoints were not working
on Windows when the inferior was multi-threaded.  This happened
because the debug registers were only updated for certain threads --
in particular, those that were being resumed and that were not marked
as suspended.  In the case of single-stepping, the need to update the
debug registers in other threads could also be "forgotten".

This patch changes windows-nat.c to mark all threads needing a debug
register update.  This brings the code closer to what gdbserver does
(though, unfortunately, it still seems more complicated than needed).