The python running on Mac M1 test builders is now native ARM64,
which makes the compilation and test use the xcodebuild/ReleaseARM64
directory, not the xcodebuild/ReleaseXARM64.
Add the xcodebuild/[mode]ARM64 build directories to sharding filesets.
Change-Id: Ide074dab3d4645f1776e83dc62c02f5ded630a9e
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/218923
Reviewed-by: Daco Harkes <dacoharkes@google.com>
