After https://github.com/llvm/llvm-project/pull/80007 Fuchsia builds are
now always building cxx_shared for arm64 and x64 Linux. Ultimately, this
is because the LIBCXX_ENABLE_SHARED is not used in compiler-rt to select
the correct libc++ target, and because cxx_shared is now always defined,
it is selected as a dependency when building runtimes tests.
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Co-authored-by: Petr Hosek <phosek@google.com>