CMakeLists: Specify -Wextra on linux builds

Allows reporting more cases where logic errors may exist, such as
implicit fallthrough cases, etc.

We currently ignore unused parameters, since we currently have many
cases where this is intentional (virtual interfaces).

While we're at it, we can also tidy up any existing code that causes
warnings. This also uncovered a few bugs as well.

src/core/hle/service/vi/vi.cpp

@@ -101,8 +101,8 @@ public:
     }
 
     std::u16string ReadInterfaceToken() {
-        u32 unknown = Read<u32_le>();
-        u32 length = Read<u32_le>();
+        [[maybe_unused]] const u32 unknown = Read<u32_le>();
+        const u32 length = Read<u32_le>();
 
         std::u16string token{};