core: Make variable shadowing a compile-time error

Now that we have most of core free of shadowing, we can enable the
warning as an error to catch anything that may be remaining and also
eliminate this class of logic bug entirely.

src/core/cpu_manager.cpp

@@ -18,7 +18,7 @@
 
 namespace Core {
 
-CpuManager::CpuManager(System& system) : system{system} {}
+CpuManager::CpuManager(System& system_) : system{system_} {}
 CpuManager::~CpuManager() = default;
 
 void CpuManager::ThreadStart(CpuManager& cpu_manager, std::size_t core) {