KVM: arm64: selftests: Split get-reg-list test code

Split the arch-neutral test code out of aarch64/get-reg-list.c into
get-reg-list.c. To do this we invent a new make variable
$(SPLIT_TESTS) which expects common parts to be in the KVM selftests
root and the counterparts to have the same name, but be in
$(ARCH_DIR).

There's still some work to be done to de-aarch64 the common
get-reg-list.c, but we leave that to the next patch to avoid
modifying too much code while moving it.

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>