[infra] Enable clang-format on macOS; clang-tidy stays linux-only

clang-format now also runs on macOS using the host's clang-format
(matching output, same pinned clang), instead of only linux-x64.

clang-tidy stays linux-x64 only because runtime/tools/run_clang_tidy.dart
assumes an x64 build (tools/build.py -ax64, out/DebugX64 includes). On
hosts where a check can't run it emits a notification so the skip stays
visible.

Change-Id: I2c43d575ee954ccd4bea37cd6f53dc6bf57f8a64
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/536461
Auto-Submit: Modestas Valauskas <valauskasmodestas@gmail.com>
Reviewed-by: Slava Egorov <vegorov@google.com>
Commit-Queue: Martin Kustermann <kustermann@google.com>
Reviewed-by: Martin Kustermann <kustermann@google.com>