Kernel: Push ARCH specific ifdef's down into RegisterState functions

The non CPU specific code of the kernel shouldn't need to deal with
architecture specific registers, and should instead deal with an
abstract view of the machine. This allows us to remove a variety of
architecture specific ifdefs and helps keep the code slightly more
portable.

We do this by exposing the abstract representation of instruction
pointer, stack pointer, base pointer, return register, etc on the
RegisterState struct.

Kernel/Thread.h

@@ -108,6 +108,24 @@ struct ThreadRegisters {
     FlatPtr rflags;
 #endif
     FlatPtr cr3;
+
+    FlatPtr ip() const
+    {
+#if ARCH(I386)
+        return eip;
+#else
+        return rip;
+#endif
+    }
+
+    FlatPtr sp() const
+    {
+#if ARCH(I386)
+        return esp;
+#else
+        return rsp;
+#endif
+    }
 };
 
 class Thread