arm64: install user ptrauth keys at kernel exit time

As we're going to enable pointer auth within the kernel and use a
different APIAKey for the kernel itself, then move the user APIAKey
switch to EL0 exception return.

The other 4 keys could remain switched during task switch, but are also
moved to keep things simple.

Reviewed-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Kristina Martšenko <kristina.martsenko@arm.com>