[ia32,root] Allow read-only ebx access in VerifyRootRegister

TBR=sigurds@chromium.org

Bug: v8:6666
Change-Id: I6fffc2187f7d457bfefe34cc78ad304cfb0a34a7
Reviewed-on: https://chromium-review.googlesource.com/1237678Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Sigurd Schneider <sigurds@chromium.org>
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#56124}

src/ia32/macro-assembler-ia32.h

@@ -119,6 +119,7 @@ class V8_EXPORT_PRIVATE TurboAssembler : public TurboAssemblerBase {
 
   void VerifyRootRegister() {
     if (FLAG_ia32_verify_root_register && FLAG_embedded_builtins) {
+      Assembler::AllowExplicitEbxAccessScope read_only_access(this);
       Label root_register_ok;
       cmp(kRootRegister, kRootRegisterSentinel);
       j(equal, &root_register_ok);