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.