arm64: KVM: Allow for direct call of HYP functions when using VHE

When running VHE, there is no need to jump via some stub to perform
a "HYP" function call, as there is a single address space.

Let's thus change kvm_call_hyp() and co to perform a direct call
in this case. Although this results in a bit of code expansion,
it allows the compiler to check for type compatibility, something
that we are missing so far.

Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>
Acked-by: Christoffer Dall <christoffer.dall@arm.com>
Signed-off-by: Christoffer Dall <christoffer.dall@arm.com>