mac: Building with --cpu=darwin_arm64 generates x86_64 binaries

[magreenblatt]

Building on MacOS 14.7 (Apple M1) with Xcode 16.2.

Steps to reproduce:

1. Build main branch with bazelisk build --config=goma //:artifacts_tar --cpu=darwin_arm64
2. Check the resulting binary architecture:

% file /private/var/tmp/_bazel_marshall/15a38429f9b5a1eebde75709bdec5b6b/execroot/_main/bazel-out/darwin_arm64-opt/bin/artifacts/rewrapper                
/private/var/tmp/_bazel_marshall/15a38429f9b5a1eebde75709bdec5b6b/execroot/_main/bazel-out/darwin_arm64-opt/bin/artifacts/rewrapper: Mach-O 64-bit executable x86_64

Actual vs expected result:

Resulting binaries should be arm64 instead of x86_64.

Additional details:

Adding the --macos_cpus=arm64 flag doesn't help.

Switching between x86_64 and arm64 bazelisk/bazel binaries (installed via Homebrew) doesn't help.

[magreenblatt]

The CiPD package for mac-arm64 includes arm64 binaries, so I'm probably just missing the correct build configuration flags.

[magreenblatt]

Adding the following in the top-level BUILD.bazel:

platform(
    name="darwin_arm64",
    constraint_values= [
        "@platforms//os:macos",
        "@platforms//cpu:arm64",
    ],
    visibility = ["//visibility:public"],
)

And then building with --cpu=darwin_arm64 --platforms=//:darwin_arm64 fixes the issue for me (generates arm64 binaries as expected).

[magreenblatt]

@gkousik It looks like you might be involved with generating the CiPD packages. Can you share the build configuration that you're using? Thanks!