Stop speculation past an eret instruction

On arm64 the eret instruction is used to return from an exception handler.
Some implementations may speculate past this instruction into the next
function. As the user may control many registers in these functions add
a synchronisation barrier sequence after the eret instruction to stop these
CPUs from speculating out of the exception handler.

PR: 242676
Submitted by: Anthony Steinhauser <asteinhauser@google.com> (previous version)
MFC after: 1 week