kpti is an optional feature, for systems not using kpti a set of
vectors for the spectre-bhb mitigations is needed.

Add another set of vectors, __bp_harden_el1_vectors, that will be
used if a mitigation is needed and kpti is not in use.

The EL1 ventries are repeated verbatim as there is no additional
work needed for entry from EL1.
Reviewed-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: James Morse <james.morse@arm.com>