core: Resolve misc cases of variable shadowing

Resolves shadowing warnings that aren't in a particularly large
subsection of core. Brings us closer to turning -Wshadow into an error.

All that remains now is for cases in the kernel (left untouched for now
since a big change by bunnei is pending), and a few left over in the
service code (will be tackled next).

src/core/tools/freezer.h

@@ -43,7 +43,7 @@ public:
     ~Freezer();
 
     // Enables or disables the entire memory freezer.
-    void SetActive(bool active);
+    void SetActive(bool is_active);
 
     // Returns whether or not the freezer is active.
     bool IsActive() const;