Rework ARC exception stack #8292
Description
Per @andrewboie in #8246:
Having 256 bytes reserved for a fatal exception stack does not sit well with me, if we go that route it will never get fixed and users will lose that RAM forever. We do have a double-fault stack for x86 but it's only 8 bytes and takes advantage of ancient i386 hardware task switching.
We agreed to merge the change in 1.12 to fix the failure and close out the release, then rework in 1.13 to reclaim the RAM.