Fortran: simplify library code for integer-to-decimal conversion
libgfortran/ChangeLog:
PR libfortran/81986
PR libfortran/99191
* libgfortran.h: Remove gfc_xtoa(), adjust gfc_itoa() and
GFC_ITOA_BUF_SIZE.
* io/write.c (write_decimal): conversion parameter is always
gfc_itoa(), so remove it. Protect from overflow.
(xtoa): Move gfc_xtoa and update its name.
(xtoa_big): Renamed from ztoa_big for consistency.
(write_z): Adjust to new function names.
(write_i, write_integer): Remove last arg of write_decimal.
* runtime/backtrace.c (error_callback): Comment on the use of
gfc_itoa().
* runtime/error.c (gfc_xtoa): Move to io/write.c.
* runtime/string.c (gfc_itoa): Take an unsigned argument,
remove the handling of negative values.