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ARROW-8420: [C++] Distinguish ARMv7 from ARMv8 in SetupCxxFlags.cmake

ARMv7 machines may not be that practical for data processing purposes, but the project should at least build there.

Closes #6914 from wesm/ARROW-8420

Authored-by: Wes McKinney <wesm+git@apache.org>
Signed-off-by: Wes McKinney <wesm+git@apache.org>