Emacs port to gcc -fcheck-pointer-bounds

[Paul Eggert <eggert@cs.ucla.edu>]

In <https://debbugs.gnu.org/29600> I published patches to port Emacs to the 
-fcheck-pointer-bounds option of GCC, so that I can debug Emacs with hardware 
pointer bounds checking on platforms that support it (such as the Kaby Lake chip 
in my year-old laptop running Ubuntu 17.10). This entails changing the 
fundamental Emacs internal word from an integer to a pointer of the same width - 
which is not as big a deal as one might think, as the commonly-used EMACS_INT 
type does not change and Emacs users and Emacs Lisp programmers should not 
notice any change.

I would like to install these patches on 'master' soon, and am mentioning this 
on emacs-devel to give a heads-up to the few but hardy volunteers who work on 
the low-level part of the Emacs implementation.