ARROW-12382: [C++] Bundle xsimd if runtime SIMD level is set
ARROW-12382: [C++] Bundle xsimd if runtime SIMD level is set

This should fix nightly Conda builds (or at least let them progress).

Closes #10029 from lidavidm/arrow-12382

Authored-by: David Li <dc24f9cfc84fb1675ce65c8444a2c3e2f0e5f297@gmail.com>
Signed-off-by: David Li <dc24f9cfc84fb1675ce65c8444a2c3e2f0e5f297@gmail.com>

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