core: Switch to unique_ptr for usage of Common::Fiber.

- With using unique_ptr instead of shared_ptr, we have more explicit ownership of the context.
- Fixes a memory leak due to circular reference of the shared pointer.

src/core/hle/kernel/svc.cpp

@@ -2626,8 +2626,7 @@ void Call(Core::System& system, u32 immediate) {
     kernel.ExitSVCProfile();
 
     if (!thread->IsCallingSvc()) {
-        auto* host_context = thread->GetHostContext().get();
-        host_context->Rewind();
+        thread->GetHostContext()->Rewind();
     }
 
     system.EnterDynarmicProfile();