Improve reporting of fatal exception on MS-Windows, to aid debugging #15994.

Improve reporting of fatal exception on MS-Windows, to aid debugging #15994.

 src/w32fns.c (my_exception_handler): New function.
 (globals_of_w32fns): Set it up as the unhandled exception
 handler.  Initialize exception code and address to zeros.
 (emacs_abort): If the exception code and address are available,
 print them at the beginning of the backtrace.  Fix the format of
 printing addresses (was producing 0x0x12345678 on XP).