fix IndicatorMax with multi-threading on ARM

[ranocha]

As reported by @rveltz on Slack.

[github-actions]

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[sloede]

Thanks!

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (9acaa90) 83.00% compared to head (b45c2e6) 80.80%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1723      +/-   ##
==========================================
- Coverage   83.00%   80.80%   -2.20%     
==========================================
  Files         431      431              
  Lines       34665    34648      -17     
==========================================
- Hits        28772    27994     -778     
- Misses       5893     6654     +761     

Flag	Coverage Δ
unittests	80.80% <100.00%> (-2.20%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files	Coverage Δ
src/solvers/dgsem_tree/indicators_1d.jl	91.83% <100.00%> (-0.75%)	⬇️
src/solvers/dgsem_tree/indicators_2d.jl	47.61% <100.00%> (+0.15%)	⬆️
src/solvers/dgsem_tree/indicators_3d.jl	96.92% <100.00%> (+0.02%)	⬆️

... and 56 files with indirect coverage changes

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