PPC/S390: Fix "unused variable" compilation error

`registers` is only used on platforms which support sparkplug.

Bug: v8:11420
Change-Id: Ia08fb1b76194db222703a64618fc2dcb00f58c96
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2780013Reviewed-by: Igor Sheludko <ishell@chromium.org>
Reviewed-by: Leszek Swirski <leszeks@chromium.org>
Commit-Queue: Milad Fa <mfarazma@redhat.com>
Cr-Commit-Position: refs/heads/master@{#73605}

src/codegen/interface-descriptors.cc

@@ -333,15 +333,15 @@ void StoreTransitionDescriptor::InitializePlatformSpecific(
 
 void BaselineOutOfLinePrologueDescriptor::InitializePlatformSpecific(
     CallInterfaceDescriptorData* data) {
+  // TODO(v8:11421): Implement on other platforms.
+#if V8_TARGET_ARCH_X64 || V8_TARGET_ARCH_ARM64 || V8_TARGET_ARCH_IA32 || \
+    V8_TARGET_ARCH_ARM
   Register registers[] = {kContextRegister,
                           kJSFunctionRegister,
                           kJavaScriptCallArgCountRegister,
                           kJavaScriptCallExtraArg1Register,
                           kJavaScriptCallNewTargetRegister,
                           kInterpreterBytecodeArrayRegister};
-  // TODO(v8:11421): Implement on other platforms.
-#if V8_TARGET_ARCH_X64 || V8_TARGET_ARCH_ARM64 || V8_TARGET_ARCH_IA32 || \
-    V8_TARGET_ARCH_ARM
   data->InitializePlatformSpecific(kParameterCount - kStackArgumentsCount,
                                    registers);
 #else