Make supervisor stack inaccessible to user mode
Previously the supervisor stack was read-only in user mode, but since the supervisor stack is typically empty when the CPU is in user mode, it's questionable whether any software actually makes use of this facility.
To simplify support for the long descriptor page table format (which doesn't support the user-RO + privileged-RW access mode), let's try and remove usermode SVC stack access completely.
Tested on Raspberry Pi 4