Dart SDK support for macOS arm64 (Apple Silicon)

[a-siva]

Dart SDK support for macOS arm64 (Apple Silicon):

• Fix issues in fetch dart: Building Dart SDK fails at fetch dart on Apple Silicon / DTK / macOS 11 #43488
• Build support: Dart SDK build rules are updated to build a version of the Dart SDK for macOS ARM host (runs natively without any translation environment needed for it).
• Automated testing: build bots to build and test this variant.
• Support in dart compile: AOT compiled apps fail when run on Apple Silicon #44001

[athomas]

Note, we won't have the build and test infrastructure set up until Q1, see https://crbug.com/1147828.

[mit-mit]

Related specific issue for Dart native compilation to executables: #44001

[a-siva]

Work on native compilation of Dart SDK for macOS ARM is progressing and we hope to have an initial build done in Q4, for full fledged support of builds and tests please see #42773 (comment)

[acoutts]

@a-siva is there anything we can try yet for building dart ourselves on ARM macOS? I tried following the build instructions but ran into the same issue as #43488

[mraleph]

Blocked on https://bugs.fuchsia.dev/p/fuchsia/issues/detail?id=67614 (our toolchain does not properly support Mac ARM64 target yet).