common: Move assert failure handling into a cpp file.

Advantage: Altering the handler does not need a full recompilation.
Disadvantage: noreturn is droped, so the caller is a bit slower.

We quite often run yuzu with a YOLO assertion handler. In fact, only very few
games run at all with asserts. This patch allows developers to patch the handler
without recompiling everything. The overhead of the missing "noreturn" attribute
shoul be negletable.