KVM: arm64: Substitute RANDOMIZE_BASE for HARDEN_EL2_VECTORS

The HARDEN_EL2_VECTORS config maps vectors at a fixed location on cores which
are susceptible to Spector variant 3a (A57, A72) to prevent defeating hyp
layout randomization by leaking the value of VBAR_EL2.

Since this feature is only applicable when EL2 layout randomization is enabled,
unify both behind the same RANDOMIZE_BASE Kconfig. Majority of code remains
conditional on a capability selected for the affected cores.

Signed-off-by: David Brazdil <dbrazdil@google.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20200721094445.82184-3-dbrazdil@google.com