Ignore com.apple. extended file attributes in contenthash

[tylermichael]

I'm not sure if this is the correct location to make this change, but this PR attempts to fix docker/for-mac#7636 by ignoring com.apple. extended file attributes in the file content hash.

I feel like this is an appropriate action to take because I don't believe extended file attributes are enabled in the linux Docker VM filesystem on Mac, so Mac specific file attributes should not even be taken into account when calculating content hashes.

[tonistiigi]

This should be done in the xattr loader loadXattr https://github.com/moby/buildkit/blob/master/vendor/github.com/tonistiigi/fsutil/stat_unix.go#L15 or in the client side Map function https://github.com/moby/buildkit/blob/master/vendor/github.com/tonistiigi/fsutil/filter.go#L33 passed by buildkit when running the transfer of local files.

[jsternberg]

I tried my hands at putting this into fsutil and created a PR here modeled off of this one.

I'm not sure if here or in the buildkit client is more appropriate for this functionality, but I figure that it probably doesn't overly matter that much.