[ddc] Adding support for arm64 build paths for DDB
[ddc] Adding support for arm64 build paths for DDB

- Adds a flag `-a` for specifying the architecture (only works for MacOS at the moment).
- Calls '-uname -m' when no architecture is specified on MacOS.
  - uname values from: https://en.wikipedia.org/wiki/Uname

Change-Id: Id5eb530e6571d2de452acd90d3b0d7e24fb32747
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/239468
Reviewed-by: Nicholas Shahan <03847abd2d8aadfe859ecb98095a31f9f515dc47@google.com>
Commit-Queue: Mark Zhou <654f5fe0c4ca6c799b0e735dcf3fc1c389f35bf9@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