windows: Prefix thread ID values with "0x" in debug traces

windows: Prefix thread ID values with "0x" in debug traces

The windows-nat.c debug traces print the thread ID in base 16,
but give no indication of it. So, in a trace like the following...

    gdb: kernel event for pid=4816 tid=720 code=CREATE_THREAD_DEBUG_EVENT)

... where tid is "720", it's easy to be confused and think that
the thread ID is 720 rather than 0x720.  This patch avoids the
confusion by adding the usual "0x" prefix used for hexadecimal
values.

gdb/ChangeLog:

        * windows-nat.c (windows_continue): Add "0x" prefix for thread
        ID in debug trace.
        (get_windows_debug_event): Likewise, for all debug traces.