Change "xor" name in cpu_core to allow building with iso646.h or C++ compiler

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`.

sim/common/ChangeLog:

	* sim-core.h (sim_cpu_core): Rename cpu_core.xor to
	cpu_core.byte_xor.
	* sim-core.c (sim_core_set_xor): Likewise.
	(sim_core_xor_read_buffer): Likewise.
	(sim_core_xor_write_buffer): Likewise.