UserspaceEmulator: Add a very simple instruction fetch cache

To avoid MMU region lookup on every single instruction fetch, we now
cache a raw pointer to the current instruction. This gets automatically
invalidated when we jump somewhere, but as long as we're executing
sequentially, instruction fetches will hit the cache and bypass all
the region lookup stuff.

This is about a ~2x speedup. :^)

DevTools/UserspaceEmulator/SimpleRegion.h

@@ -45,6 +45,8 @@ public:
 
     u8* data() { return m_data; }
 
+    virtual u8* cacheable_ptr(u32 offset) override;
+
 private:
     u8* m_data { nullptr };
 };