armv8: Disable exception vectors in SPL by default

Commit 1416e2d2 ("armv8: make SPL exception vectors optional") had a
typo in it which effectively disabled exception handling in SPL code always.

Since nobody complained, I guess we may as well disable exception handling
in SPL always by default.

So fix the bug to make the config option effective, but disable exception
handling in SPL by default. This gets us to the same functionality as before
by default, but with much less code included in the binary.

Signed-off-by: Alexander Graf <agraf@suse.de>
Reviewed-by: Matthias Brugger <mbrugger@suse.com>
Reviewed-by: Andre Przywara <andre.przywara@arm.com>