tests: fix portability issue in dd/ascii test

Solaris 8 was seen to issue this error:
"printf: `&': illegal format character"

* test/dd/ascii.sh: Use the coreutils printf in this test
rather than the system one, to avoid portability issues.
rm: fix prompted number of arguments to remove on some platforms

"zu" was output on solaris 8 for example rather than the number,
since coreutils-8.22.

* cfg.mk: Disallow %z, since we don't currently use the gnulib
fprintf module, so any usage with it is non portable.  Also
our usage with error() currently works only through an ancillary
dependency on the vfprintf gnulib module.
* src/rm.c (main): Use %PRIuMAX rather than %zu for portability.
* src/dd.c (alloc_[io]buf): Likewise for consistency.
* src/od.c (main): Likewise.
* src/split.c (set_suffix_length): Likewise.
* NEWS: Mention the rm bug fix.
Reported in http://bugs.gnu.org/19184