Revert "[embedded-builtins] Enable on all arches except x86 for benchmarks"

This reverts commit f5a8352b.

Reason for revert: Performance issues

Original change's description:
> [embedded-builtins] Enable on all arches except x86 for benchmarks
> 
> This CL enables embedded builtins to get benchmark feedback. We need
> this feedback to identify and address remaining performance problems.
> 
> Bug: v8:6666
> Change-Id: I8f77f218e656b55ddabe1236eb2a1d14a5ac6233
> Reviewed-on: https://chromium-review.googlesource.com/1105834
> Reviewed-by: Jakob Gruber <jgruber@chromium.org>
> Commit-Queue: Sigurd Schneider <sigurds@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#53836}

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

# Not skipping CQ checks because original CL landed > 1 day ago.

Bug: v8:6666
Change-Id: I0e0897eefa069b0b9ad2dd56b2ffc3e3617f9258
Reviewed-on: https://chromium-review.googlesource.com/1139974
Commit-Queue: Sigurd Schneider <sigurds@chromium.org>
Reviewed-by: Sigurd Schneider <sigurds@chromium.org>
Cr-Commit-Position: refs/heads/master@{#54498}

BUILD.gn

@@ -77,8 +77,10 @@ declare_args() {
 
   # Enable embedded builtins.
   # TODO(jgruber,v8:6666): Support ia32 and maybe MSVC.
+  # TODO(jgruber,v8:6666): Enable for remaining architectures once performance
+  #                        regressions are addressed.
   v8_enable_embedded_builtins =
-      v8_use_snapshot && v8_current_cpu != "x86" && (!is_win || is_clang)
+      v8_use_snapshot && v8_current_cpu == "x64" && (!is_win || is_clang)
 
   # Enable code-generation-time checking of types in the CodeStubAssembler.
   v8_enable_verify_csa = false