KVM: arm64: Save chosen hyp vector to a percpu variable

Introduce a percpu variable to hold the address of the selected hyp
vector that will be used with guests. This avoids the selection process
each time a guest is being entered and can be used by nVHE when a
separate vector is introduced for the host.

Signed-off-by: Andrew Scull <ascull@google.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20200915104643.2543892-6-ascull@google.com