getprogname: Add support for ASCII-compatible environments in z/OS. Reported by Mike Fulton <mikefultonpersonal@gmail.com> in <https://lists.gnu.org/archive/html/bug-gnulib/2023-05/msg00198.html>. * lib/getprogname.c (getprogname): On z/OS, when compiling for an ASCII-compatible environment, convert the result from EBCDIC to ASCII.