target-arm: fix build with gcc 4.8.2

commit 5ce4f35781
    "target-arm: A64: add set_pc cpu method"

introduces an array aarch64_cpus which is zero
size if this code is built without CONFIG_USER_ONLY.
In particular an attempt to iterate over this array produces a warning
under gcc 4.8.2:

 CC    aarch64-softmmu/target-arm/cpu64.o
/scm/qemu/target-arm/cpu64.c: In function ‘aarch64_cpu_register_types’:
/scm/qemu/target-arm/cpu64.c:124:5: error: comparison of unsigned
expression < 0 is always false [-Werror=type-limits]
     for (i = 0; i < ARRAY_SIZE(aarch64_cpus); i++) {
     ^
cc1: all warnings being treated as errors

This is the result of ARRAY_SIZE being an unsigned type,
causing "i" to be promoted to unsigned int as well.

As zero size arrays are a gcc extension, it seems
cleanest to add a dummy element with NULL name,
and test for it during registration.

We'll be able to drop this when we add more CPUs.

Cc: Alexander Graf <agraf@suse.de>
Cc: Peter Maydell <peter.maydell@linaro.org>
Cc: Richard Henderson <rth@twiddle.net>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Stefan Weil <sw@weilnetz.de>

target-arm/cpu64.c

@@ -58,6 +58,7 @@ static const ARMCPUInfo aarch64_cpus[] = {
 #ifdef CONFIG_USER_ONLY
     { .name = "any",         .initfn = aarch64_any_initfn },
 #endif
+    { .name = NULL } /* TODO: drop when we support more CPUs */
 };
 
 static void aarch64_cpu_initfn(Object *obj)
@@ -100,6 +101,11 @@ static void aarch64_cpu_register(const ARMCPUInfo *info)
         .class_init = info->class_init,
     };
 
+    /* TODO: drop when we support more CPUs - all entries will have name set */
+    if (!info->name) {
+        return;
+    }
+
     type_info.name = g_strdup_printf("%s-" TYPE_ARM_CPU, info->name);
     type_register(&type_info);
     g_free((void *)type_info.name);