selftests/kvm: Fix issues with $(SPLIT_TESTS)

The introduction of $(SPLIT_TESTS) also introduced a warning when
building selftests on architectures that include get-reg-lists:

    make: Entering directory '/root/kvm/tools/testing/selftests/kvm'
    Makefile:272: warning: overriding recipe for target '/root/kvm/tools/testing/selftests/kvm/get-reg-list'
    Makefile:267: warning: ignoring old recipe for target '/root/kvm/tools/testing/selftests/kvm/get-reg-list'
    make: Leaving directory '/root/kvm/tools/testing/selftests/kvm'

In addition, the rule for $(SPLIT_TESTS_TARGETS) includes _all_
the $(SPLIT_TESTS_OBJS), which only works because there is just one.
So fix both by adjusting the rules:

- remove $(SPLIT_TESTS_TARGETS) from the $(TEST_GEN_PROGS) rules,
  and rename it to $(SPLIT_TEST_GEN_PROGS)

- fix $(SPLIT_TESTS_OBJS) so that it plays well with $(OUTPUT),
  rename it to $(SPLIT_TEST_GEN_OBJ), and list the object file
  explicitly in the $(SPLIT_TEST_GEN_PROGS) link rule

Fixes: 17da79e0 ("KVM: arm64: selftests: Split get-reg-list test code", 2023-08-09)
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Tested-by: Andrew Jones <ajones@ventanamicro.com>
Reviewed-by: Marc Zyngier <maz@kernel.org>
Signed-off-by: Anup Patel <anup@brainfault.org>