doc: mention intptr_t etc. and IBM i * doc/gnulib-readme.texi (Other portability assumptions): Mention intptr_t and uintptr_t, and that arithmetic on them works in the usual way. (Exotic platforms): New section, containing material from the old 'Integer Portability' section. Also mention IBM i. * doc/intprops.texi (Wraparound Arithmetic): Say that the macros work on unsigned integers too. (Integer Portability): Remove.