KVM: arm64: selftests: Finish generalizing get-reg-list

Add some unfortunate #ifdeffery to ensure the common get-reg-list.c
can be compiled and run with other architectures. The next
architecture to support get-reg-list should now only need to provide
$(ARCH_DIR)/get-reg-list.c where arch-specific print_reg() and
vcpu_configs[] get defined.

Signed-off-by: Andrew Jones <ajones@ventanamicro.com>
Signed-off-by: Haibo Xu <haibo1.xu@intel.com>
Signed-off-by: Anup Patel <anup@brainfault.org>