Fix kArchRootsPointer typo on s390

TBR=jarin@chromium.org

Bug: v8:6666
Change-Id: I9f4654955f297c54e67b98afb42125b659e35fd3
Reviewed-on: https://chromium-review.googlesource.com/971042Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#52074}

src/compiler/s390/code-generator-s390.cc

@@ -1325,7 +1325,7 @@ CodeGenerator::CodeGenResult CodeGenerator::AssembleArchInstruction(
         __ LoadRR(i.OutputRegister(), fp);
       }
       break;
-    case kArchFramePointer:
+    case kArchRootsPointer:
       __ LoadRR(i.OutputRegister(), kRootRegister);
       break;
     case kArchTruncateDoubleToI: