* 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.