x86: Fix cpuid family/model composition

Fixes a bug where previously MODEL_ID() was defined as:
    `#define MODEL_ID(x) ( ((x & 0xf0000) >> 16) + (x & 0xf0) )`

This was incorrect because (1) it didn't take into account the conditional
nature of the extended_model_ID, and (2) it's actually shifting the
extended_model_ID into the low bits and keeping the model_ID in the high bits,
when it should be the other way around.

This patch also introduces a foundation for more sane testing of CPU vendor,
family, model and brand_ID.

include/util.h

@@ -39,6 +39,7 @@
 #define USED         __attribute__((used))
 #define FASTCALL     __attribute__((fastcall))
 #define UNREACHABLE()  __builtin_unreachable()
+#define MAY_ALIAS    __attribute__((may_alias))
 
 #define OFFSETOF(type, member) \
     __builtin_offsetof(type, member)