s390x: adjust kHeaderPaddingSize

PPC/S390 has been droped 32/31-bit support.
So adjust the padding size for 64-bit only.

Change-Id: I3533ef4a90bee0b1e6f49aeb61498ce3054e85e7
Reviewed-on: https://chromium-review.googlesource.com/c/1397866Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Commit-Queue: Junliang Yan <jyan@ca.ibm.com>
Cr-Commit-Position: refs/heads/master@{#58598}

src/objects/code.h

@@ -403,11 +403,11 @@ class Code : public HeapObject {
 #elif V8_TARGET_ARCH_MIPS
   static constexpr int kHeaderPaddingSize = 20;
   STATIC_ASSERT(kHeaderSize - kHeaderPaddingStart == kHeaderPaddingSize);
-#elif V8_TARGET_ARCH_PPC
+#elif V8_TARGET_ARCH_PPC64
   // No static assert possible since padding size depends on the
   // FLAG_enable_embedded_constant_pool runtime flag.
-#elif V8_TARGET_ARCH_S390
-  static constexpr int kHeaderPaddingSize = 20;
+#elif V8_TARGET_ARCH_S390X
+  static constexpr int kHeaderPaddingSize = 0;
   STATIC_ASSERT(kHeaderSize - kHeaderPaddingStart == kHeaderPaddingSize);
 #else
 #error Unknown architecture.