add --nan-equal option for ART

Julien Maurerrequested to merge
jmaurer-21.0-mc16d-patch-09815 into 21.0-mc16d

Manual cherry-pick of 5e5a5c97 in order to fix Tier0ChainTest ART comparisons in this branch which are broken since... a few years I guess.

Edited by Julien Maurer

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