KVM: arm64: Use build-time defines in has_vhe()

Build system compiles hyp code with macros specifying if the code belongs
to VHE or nVHE. Use these macros to evaluate has_vhe() at compile time.

Signed-off-by: David Brazdil <dbrazdil@google.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20200625131420.71444-5-dbrazdil@google.com