* dcrt0.cc (dll_crt0_0): Install myfault exception handler on x86_64.

* exception.h (exception_list): Typedef as void on x86_64.
	(exception::handler_installed): Remove.
	(exception::handle_while_being_debugged): Remove.
	(exception::myfault_handle): Declare for x86_64.
	(exception::handle): Declare as ordinary exception handler on x86_64
	as well.
	(exception::exception): Drop previous code (again).  Install
	exception::handle as SEH handler.
	(exception::install_myfault_handler): New x86_64-only method to
	install exception::myfault_handle as VEH handler.  Explain why.
	(exception::~exception): For x86_64, define frame end label (again).
	* exceptions.cc (CYG_EXC_CONTINUE_EXECUTION): Drop definition.
	(CYG_EXC_CONTINUE_SEARCH): Ditto.
	(exception::myfault_handle): New x86_64-only method, VEH handler to
	handle myfault exceptions.
	(exception::handle): Define as ordinary exception handler on x86_64
	as well.  Use ExceptionContinueExecution and ExceptionContinueSearch
	throughout instead of deleted Cygwin macros.  Don't handle myfault
	exceptions on x86_64.

winsup/cygwin/dcrt0.cc

@@ -795,6 +795,10 @@ dll_crt0_0 ()
 
   _main_tls = &_my_tls;
 
+#ifdef __x86_64__
+  exception::install_myfault_handler ();
+#endif
+
   /* Initialize signal processing here, early, in the hopes that the creation
      of a thread early in the process will cause more predictability in memory
      layout for the main thread. */