Revert "[deps] No longer need mac-x64 on mac-arm64."

This reverts commit c129f3f.

Reason for revert: still used by x64-sdk builder which runs on an arm64 machine

Original change's description:
> [deps] No longer need mac-x64 on mac-arm64.
>
> Goma made use of mac-x64, but RBE uses linux-x64.
>
> Change-Id: I7837aa0f48649e6738a78f89067814e8fc116908
> Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/446703
> Reviewed-by: Alexander Aprelev <[email protected]>
> Commit-Queue: Ryan Macnak <[email protected]>

No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Change-Id: Idcd436d7436b4b9d43d1cc0c08cd314467273764
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/447000
Auto-Submit: Ryan Macnak <[email protected]>
Reviewed-by: Alexander Aprelev <[email protected]>
Commit-Queue: Ryan Macnak <[email protected]>

Author: rmacnak-google

DEPS

@@ -439,7 +439,7 @@ deps = {
               "version": Var("clang_version"),
           },
       ],
-      "condition": "host_os == 'mac' and host_cpu == 'x64'",
+      "condition": "host_os == mac", # On ARM64 Macs too because Goma doesn't support the host-arm64 toolchain.
       "dep_type": "cipd",
   },
   Var("dart_root") + "/buildtools/win-x64/clang": {