Change "xor" name in cpu_core to allow building with iso646.h or C++ compiler
This code was introduced back in 1998, and C99 appeared next year,
defining the `xor` as an 'alternative keyword' if iso646.h is
included. Moreover, C++98 defines it on the language level. As a
result, the code is not buildable with C++ compiler or if iso646.h is
included beforehand.
According to comment, `sim_cpu_core` is mostly a clone of `sim_core`,
so I renamed it to `byte_xor` as it's called in `sim_core`.