[macOS][gn] Use arm64 clang in sanitizer_suppressions.sh on arm64 Macs

[cbracken]

This eliminates instances of clang host toolchain path that assumes clang-x64 on macOS hosts and instead uses the toolchain for the host architecture.

Note that if the build is intentionally invoked under Rosetta 2 -- for example by running arch -x86_64 BUILD_COMMAND, host_cpu will evaluate to x86_64. As such, it is still possible to run builds using the x64 toolchain on demand, but the default behaviour is now to build consistently with the appropriate clang toolchain for the host CPU architecture.

No test changes because the build (and existing tests) are the test.

Issue: #103386

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Code Review

This pull request updates the sanitizer suppressions script to support arm64 architecture on macOS by dynamically setting the build tools directory. Feedback suggests refactoring the architecture detection logic to be self-contained within the Darwin block to prevent potential conflicts with global environment variables and to remove unnecessary architecture checks.

[vashworth]

According to this comment, it could also be something like arm64_v8a. Perhaps compare against x86_64 and then default to arm64?

flutter/packages/flutter_tools/lib/src/base/os.dart

Lines 268 to 270 in 54e3e36

	final RunResult hostPlatformCheck = _processUtils.runSync(<String>['uname', '-m']);
	// On x64 stdout is "uname -m: x86_64"
	// On arm64 stdout is "uname -m: aarch64, arm64_v8a"

[cbracken]

See comment here for discussion:

• [macOS][gn] Use arm64 clang in copy_info_plist.py on arm64 Macs #186661 (comment)
• [macOS][gn] Use arm64 clang in generate_coverage.py on arm64 Macs #186662 (comment)