Windows gdb+gdbserver: Move suspending thread to when returning event

The current code suspends a thread just before calling
GetThreadContext.  You can only call GetThreadContext if the thread is
suspended.  But, after WaitForDebugEvent, all threads are implicitly
suspended.  So I don't think we even needed to call SuspendThread
explictly at all before our GetThreadContext calls.

However, suspending threads when we're about to present a stop to gdb
simplifies adding non-stop support later.  This way, the windows
SuspendThread state corresponds to whether a thread is suspended or
resumed from the core's perspective.  Curiously, I noticed that Wine's
winedbg does something similar:
https://github.com/wine-mirror/wine/blob/234943344f7495d1e072338f0e06fa2d5cbf0aa1/programs/winedbg/gdbproxy.c#L651

This makes it much easier to reason about a thread's suspend state,
and simplifies adding non-stop mode later on.

Approved-By: Tom Tromey <tom@tromey.com>
Change-Id: Ifd6889a8afc041fad33cd1c4500e38941da6781b
commit-id:c4d2c92e