add full translation support for exit stubs, which are used for aarch64 steady-state code

Xref #4669 which is suggesting adding both pc and full xl8 support to fragment prefixes, which are on every block for aarch64, and are on ib targets for x86.

Today we fail full-state xl8 up front for exit stubs; pc xl8 succeeds.
For x86 this is fine since we only use stubs for unlinked direct branches: indirect go straight to the ibl, so hot code is never going to be in a stub.
But for aarch64, we're frequently linking through stubs for reachability, so stub xl8 matters for synchall: by not having it we reduce the performance of attach, detach, synchall flushes, etc.