Fix Linux AArch64 build and add CI for that platform (#7273)

The `--dump-cpp-ast` file tests strip references to Clang builtins so
that the expected output is target-independent. The filter anchored a
`__`-prefixed builtin identifier on a preceding space or quote, which
matches the x86-64 `__va_list_tag` spelling but not the AArch64
`std::__va_list`, where `__` is preceded by the `::` namespace
qualifier. That left a single `RecordType 'std::__va_list'` line
unfiltered on AArch64, producing a spurious autoupdate diff for
`thunk_ast.carbon`.

Anchor the match on a preceding `:` as well so namespace-qualified
builtins are also filtered.

Carbon's test workflow covered Linux on x86-64 and macOS on AArch64, but
had no Linux AArch64 coverage, so AArch64-specific issues that don't
reproduce on macOS could land unnoticed. Add an `ubuntu-22.04-arm`
runner to the matrix.

The release used for Linux does not publish the monolithic
`LLVM-*-Linux-ARM64` package, only a `clang+llvm-*-aarch64-linux-gnu`
community build with a smaller tool set, so the Ubuntu setup now selects
the tarball by `runner.arch`. The prune step uses `rm -f` since the two
packages do not ship an identical set of tools to remove.

Assisted-by: Claude Code with Claude Opus 4.7

.github/workflows/tests.yaml

@@ -27,8 +27,9 @@ jobs:
       matrix.config.name) || '' }} (${{ matrix.runner }})
     strategy:
       matrix:
-        # Test a recent version of each supported OS.
-        runner: ['ubuntu-22.04', 'macos-14']
+        # Test a recent version of each supported OS, covering both x86-64 and
+        # AArch64: Linux on x86-64 and AArch64, and macOS on AArch64.
+        runner: ['ubuntu-22.04', 'ubuntu-22.04-arm', 'macos-14']
         # Create a synthetic matrix dimension with the event name for filtering.
         event: ['${{ github.event_name }}']
         config: