arm64: KVM: remove __kvm_hyp_code_{start,end} from hyp.S

We already have __hyp_text_{start,end} to express the boundaries
of the HYP text section, and __kvm_hyp_code_{start,end} are getting
in the way of a more modular world switch code.

Just turn __kvm_hyp_code_{start,end} into #defines mapping the
linker-emited symbols.

Acked-by: Catalin Marinas <catalin.marinas@arm.com>
Reviewed-by: Christoffer Dall <christoffer.dall@linaro.org>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>