arm64: Allow exceptions to return

Our current arm64 exception handlers all panic and never return to the
exception triggering code.

But if any handler wanted to continue execution after fixups, it would
need help from the exception handling code to restore all registers.

This patch implements that help. With this code, exception handlers on
aarch64 can successfully return to the place the exception happened (or
somewhere else if they modify elr).

Signed-off-by: Alexander Graf <agraf@suse.de>