Add ClearAllMemChecks to DebugInterface

Breakpoints have one, but memchecks don't, despite being cleared directly in the breakpoint window.

Now DolphinWX should call the interface functions and not the direct functions of the breakpoints or memchecks for clearing.
Mark functions in the PPCDebugInterface and DSPDebugInterface as final.

The way they currently were, they would allow for a class to inherit and override these interfaces functions. However, this doesn't make sense conceptually.