GCC should treat plain char in the same fashion on all types of machines (by default).
The ISO C standard leaves it up to the implementation whether a char declared plain char is signed or not. This in effect creates two alternative dialects of C. The GNU C compiler supports both dialects; you can specify the signed dialect with -fsigned-char and the unsigned dialect with -funsigned-char. However, this leaves open the question of which dialect to use by default. The preferred dialect makes plain char signed, because this is simplest. Since int is the same as signed int, short is the same as signed short, etc., it is cleanest for char to be the same. Some computer manufacturers have published Application Binary Interface standards which specify that plain char should be unsigned. It is a mistake, however, to say anything about this issue in an ABI. This is because the handling of plain char distinguishes two dialects of C. Both dialects are meaningful on every type of machine. Whether a particular object file was compiled using signed char or unsigned is of no concern to other object files, even if they access the same chars in the same data structures. A given program is written in one or the other of these two dialects. The program stands a chance to work on most any machine if it is compiled with the proper dialect. It is unlikely to work at all if compiled with the wrong dialect. Many users appreciate the GNU C compiler because it provides an environment that is uniform across machines. These users would be inconvenienced if the compiler treated plain char differently on certain machines. Occasionally users write programs intended only for a particular machine type. On these occasions, the users would benefit if the GNU C compiler were to support by default the same dialect as the other compilers on that machine. But such applications are rare. And users writing a program to run on more than one type of machine cannot possibly benefit from this kind of compatibility. There are some arguments for making char unsigned by default on all machines. If, for example, this becomes a universal de facto standard, it would make sense for GCC to go along with it. This is something to be considered in the future. (Of course, users strongly concerned about portability should indicate explicitly whether each char is signed or not. In this way, they write programs which have the same meaning in both C dialects.)
