Try fixing unused-variable warnings another way.

If we mark these as extern, the compiler won't complain they're unused.

This is an unsatisfying hack, but last time we tried to condition these
on non-NEON only, Chromium's GN linux ARM bot failed.  They must not be
configured to use the NEON procs?

BUG=skia:4798
GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1699473002
CQ_EXTRA_TRYBOTS=client.skia:Test-Ubuntu-GCC-GCE-CPU-AVX2-x86_64-Release-SKNX_NO_SIMD-Trybot

Review URL: https://codereview.chromium.org/1699473002
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