Windows gdb+gdbserver: Eliminate thread_rec(INVALIDATE_CONTEXT) calls

Windows gdb+gdbserver: Eliminate thread_rec(INVALIDATE_CONTEXT) calls

Replace thread_rec(INVALIDATE_CONTEXT) calls with find_thread, and
invalidate_context / suspend calls in the spots that might need those.

I don't know why does the INVALIDATE_CONTEXT implementation in GDB
avoid suspending the event thread:

	case INVALIDATE_CONTEXT:
	  if (ptid.lwp () != current_event.dwThreadId)
	    th->suspend ();

Checks for a global "current_event" get in the way of non-stop support
later in the series, as each thread will have its own "last debug
event".  Regardless, it should be fine to suspend the event thread.
As a data point, the GDBserver implementation always suspends.  So
this patch does not try to avoid suspending the event thread on the
native side either.

Approved-By: Tom Tromey <tom@tromey.com>
Change-Id: I8d2f0a749d23329956e62362a7007189902dddb5