Fix target_cpu for "arm_x64" to be "arm" not "x64".
TEST=ci
Change-Id: Ifebaee036f3dfa5ba3824f05472423a5bcf9c0c0
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/290360
Commit-Queue: Ryan Macnak <rmacnak@google.com>
Reviewed-by: Daco Harkes <dacoharkes@google.com>