kernel/resource_limit: Clean up interface

Cleans out the citra/3DS-specific implementation details that don't
apply to the Switch. Sets the stage for implementing ResourceLimit
instances properly.

While we're at it, remove the erroneous checks within CreateThread() and
SetThreadPriority(). While these are indeed checked in some capacity,
they are not checked via a ResourceLimit instance.

In the process of moving out Citra-specifics, this also replaces the
system ResourceLimit instance's values with ones from the Switch.