The phrasing "damage can’t be reduced or prevented in any way" should supercede relentless endurance
Relentless Endurance. When you are reduced to
0 hit points but not killed outright, you can drop to 1 hit point instead. You can’t use this feature again until you finish a long rest.
The relevant part of wish says:
After enduring that stress, each time you cast a spell until you finish a long rest, you take 1d10 necrotic damage per level of that spell. This damage can’t be reduced or prevented in any way.
While this is a case of two specific effects conflicting, I think that the wording of wish makes it clear that it should supercede the wording of relentless endurance.
The damage from wish is explicitly and non-optionally excluded from any effects that would reduce or prevent the damage. However, the wording of relentless endurance makes it an optional ability. Requirements should supercede something that is optional.
The wording even says "in any way". This kind of emphasis (which is not necessary for the spell's wording and not usually put in there) also implies that it should be considered first and that it overrides any other effects.
There is also the fact that wish is an incredibly powerful 9th level spell. Allowing a racial feature to override it seems like a gross power imbalance.
For example, if the orc wizard casting wish was at 1 hp at the time that the damaging effect of wish was triggered (and the effect did not outright kill them) then relentless endurance would allow essentially completely nullify the damage.
In the end, this is a specific vs specific effect that does not have a truly explicit resolution. Thus, the DM will have to make the final call. However, I think, all things considered, that relentless endurance should clearly not be allowed to reduce damage from effects like these.