arm64: Have a common call to userret

[andrew]

Summary

Rather than each exception calling userret use a common copy. As
syscallret already calls userret we need to skip it in that case.

Sponsored by: Arm Ltd

[kib]

IMHO the control flow could be made less confusing. Right now it is 'break' -> userret, and in one place it is goto no_userret. I suggest to add a control variable like 'use_userret', set to true, assigned from svc_handler() return value. Then instead of goto no userret. check the value of use_userret around userret() call. Hope it is understandable.