arm64: entry: Make the kpti trampoline's kpti sequence optional

Spectre-BHB needs to add sequences to the vectors. Having one global
set of vectors is a problem for big/little systems where the sequence
is costly on cpus that are not vulnerable.

Making the vectors per-cpu in the style of KVM's bh_harden_hyp_vecs
requires the vectors to be generated by macros.

Make the kpti re-mapping of the kernel optional, so the macros can be
used without kpti.

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>