PPC: [ptr-compr] Prepare for changing kTaggedSize, pt.1

Port f70bb59b

Original Commit Message:

    Includes various fixes and cleanups here and there.

R=ishell@chromium.org, joransiu@ca.ibm.com, michael_dawson@ca.ibm.com, miladfar@ca.ibm.com

LOG=N

Bug: v8:7703, v8:8852
Change-Id: I1e25c6da6677aab64dabc052aee555f534386df7
Reviewed-on: https://chromium-review.googlesource.com/c/1491698Reviewed-by: Igor Sheludko <ishell@chromium.org>
Commit-Queue: Junliang Yan <jyan@ca.ibm.com>
Cr-Commit-Position: refs/heads/master@{#59922}

src/objects/code.h

@@ -414,7 +414,8 @@ class Code : public HeapObject {
 #elif V8_TARGET_ARCH_MIPS
   static constexpr int kHeaderPaddingSize = 20;
 #elif V8_TARGET_ARCH_PPC64
-  static constexpr int kHeaderPaddingSize = 0;
+  static constexpr int kHeaderPaddingSize =
+      FLAG_enable_embedded_constant_pool ? 28 : 0;
 #elif V8_TARGET_ARCH_S390X
   static constexpr int kHeaderPaddingSize = 0;
 #else