x86: parse_real_register() does not alter the parsed string

x86: parse_real_register() does not alter the parsed string

Follow the model of strtol() et al - input string is const-qualified to
signal that the string isn't altered, but the returned "end" pointer is
not const-qualified, requiring const to be cast away (which generally is
a bad idea, but the alternative would be more convoluted code).