* cygtls.h (struct _cygtls): Convert thread_context to type CONTEXT.
* exceptions.cc (_cygtls::signal_debugger): Use sizeof (CONTEXT) for
size of CONTEXT copied for GDB's digestion.
* include/cygwin/signal.h: Add a preliminary comment.
Signed-off-by: Corinna Vinschen <corinna@vinschen.de>