[patch] Fix PR libfortran/95177, ctype.h functions should not be called with char arguments

[FX]

Hi,

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.

Bootstrapped and regtested on x86_64-pc-linux-gnu.
OK to commit?

FX

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