Fortran: Cast arguments of <ctype.h> functions to unsigned char

Functions from <ctype.h> should only be called on values that can be
represented by unsigned char. On targets where char is a signed type,
some of libgfortran calls have undefined behaviour.

The solution is to cast the argument to unsigned char type. I’ve defined
macros in libgfortran.h to do so, to retain legibility of the library
code.

PR libfortran/95177

libgfortran/ChangeLog

	* libgfortran.h: include ctype.h, provide safe macros.
	* io/format.c: use safe macros.
	* io/list_read.c: use safe macros.
	* io/read.c: use safe macros.
	* io/write.c: use safe macros.
	* runtime/environ.c: use safe macros.