kernel/svc: Deglobalize the supervisor call handlers

Adjusts the interface of the wrappers to take a system reference, which
allows accessing a system instance without using the global accessors.

This also allows getting rid of all global accessors within the
supervisor call handling code. While this does make the wrappers
themselves slightly more noisy, this will be further cleaned up in a
follow-up. This eliminates the global system accessors in the current
code while preserving the existing interface.