[misc] Generalize .gitignore for sysroots
[misc] Generalize .gitignore for sysroots

Change-Id: I62601b0e458250da3a94764d2577d586ab8e5e1d
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/255249
Reviewed-by: Ryan Macnak <d738b7f001baf9789095d7dc549cff6de60b011d@google.com>
Commit-Queue: Daco Harkes <930cbae0ca77f1954c53e32620d89cfcca318f00@google.com>

dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk