Revert "Fix kArchRootsPointer typo on s390"

This reverts commit b105df6d.

Reason for revert:
https://build.chromium.org/p/client.v8.ports/builders/V8%20Arm/builds/6616

Original change's description:
> Fix kArchRootsPointer typo on s390
> 
> TBR=jarin@chromium.org
> 
> Bug: v8:6666
> Change-Id: I9f4654955f297c54e67b98afb42125b659e35fd3
> Reviewed-on: https://chromium-review.googlesource.com/971042
> Reviewed-by: Jakob Gruber <jgruber@chromium.org>
> Commit-Queue: Jakob Gruber <jgruber@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#52074}

TBR=jarin@chromium.org,jgruber@chromium.org

Change-Id: Ibff56d207b12a0664e6ed66e5f6f4c725206e526
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: v8:6666
Reviewed-on: https://chromium-review.googlesource.com/971901Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Commit-Queue: Michael Achenbach <machenbach@chromium.org>
Cr-Commit-Position: refs/heads/master@{#52082}

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

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