Remove the runtime OR operation on the register gateway function pointer

The current implementation of the register gateway casts a read-only structure to a function pointer and executes it.

The gateway address must be OR-ed with 1 to ensure that the Thumb bit is set and that the branch to the gateway does not trigger an `INVPC` usage fault.

This OR operation at the moment needs to be performed explicitly, at runtime. The cast to a function pointer is not enough to set the Thumb bit.

Locate the register gateway implementation and inspect how the read-only structure is cast to a function pointer and how the gateway address is OR-ed with 1. The change is complete when the runtime OR operation is removed without reintroducing the INVPC usage fault.

Written by the indexing model from the issue text.