AArch64: correct epilogue/prologue emission for swift async

The prologue and epilogue emission were unbalanced in light of different
strategies of async frame context emission.  Adjust the epilogue emission
to match the prologue emission.  This makes the elision work properly as
well as the deployment based.  Due to the fact that the epilogue always
was clearing a bit (which should not be set in the first place), the
client would not notice the behavioural issue unless the deployment
version was in effect.