Fixed x86_64 handling by the build system.

* Map build variables HOST_CPU and HOST_ARCH to x86_64, if it they are
* x86 and
  64 bit and define the __x86_64__ C macro instead of __INTEL__ in that
case.
* <OS.h>: Also handle __x86_64__.

headers/os/kernel/OS.h

@@ -414,6 +414,8 @@ extern void			ktrace_vprintf(const char *format, va_list args);
 
 #if __INTEL__
 #	define B_MAX_CPU_COUNT	8
+#elif __x86_64__
+#	define B_MAX_CPU_COUNT	8
 #elif __POWERPC__
 #	define B_MAX_CPU_COUNT	8
 #elif __M68K__