kvm: arm64: Only define __kvm_ex_table for CONFIG_KVM

Minor cleanup that only creates __kvm_ex_table ELF section and
related symbols if CONFIG_KVM is enabled. Also useful as more
hyp-specific sections will be added.

Signed-off-by: David Brazdil <dbrazdil@google.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Acked-by: Will Deacon <will@kernel.org>
Link: https://lore.kernel.org/r/20200922204910.7265-4-dbrazdil@google.com