Bug 391488: Eliminate PTRDIFF macro; delete jssttddef.h. r=jimb

Delete jsstddef.h, since its only remaining purpose is to make certain
Win16-specific definitions; we don't support Win16 any more.  In
particular, we can just subtract pointers now, so the PTRDIFF macro is
unnecessary noise.

Most places get stddef.h via jstypes.h or some other header, so we can
just delete #inclusions of jstddef.h.  The exception is jskwgen.h, so
there we explicitly include <stddef.h> instead.

js/src/jsarray.cpp

@@ -75,7 +75,6 @@
  * a js_SlowArrayClass, but have the same performance characteristics as
  * a dense array for slot accesses, at some cost in code complexity.
  */
-#include "jsstddef.h"
 #include <stdlib.h>
 #include <string.h>
 #include "jstypes.h"