PR gdb/17096: async support breaks remote debugging on Windows

PR gdb/17096: async support breaks remote debugging on Windows

On Windows, with "maint set target-async on" (the default since
a09dd441), Ctrl-C fails to stop a remote target.

With maint target-async on, the SIGINT signal handler doesn't send the
remote interrupt request immediately.  Instead, it marks an async
handler as ready, and then the main event loop wakes up and notices
that the SIGINT async signal handler token was set, and calls the
corresponding event handler, which sends the remote interrupt request.

On POSIX-like systems, the SIGINT signal makes the select/poll in the
main event loop wake up / return with EINTR.  However, on Windows,
signal handlers run on a separate thread, and Windows doesn't really
have a concept of EINTR.  So, just marking the async handler
(effectively just setting a flag) does not wake up gdb_select.
Instead, we need to call gdb_call_async_signal_handler from the signal
handler.  The Windows version (in mingw-hdep.c) sets a Windows event
that gdb_select's WaitForMultipleObjects is waiting for.

Confirmed that with this, Ctrl-C interrupts the remote target on
Windows.  Also regression tested on x86_64 Fedora 20 against
GDBserver.

gdb/
2014-07-07  Pedro Alves  <palves@redhat.com>

	* remote.c (async_handle_remote_sigint)
	(async_handle_remote_sigint_twice): Call
	gdb_call_async_signal_handler instead of
	mark_async_signal_handler.