as such a bitcoin miner runs *ENTIRELY* on the GPU in OpenCL (or Cuda, but Nvidia cards have much worse performances)
the code running on the main CPU is only in charge of setting everything up and then fetch work from pool and send it to miner worker on GPU, so it doesn't have much impact on the mining process and thus 32 vs. 64 doesn't matter much.
But in fact most miners used a lot of GFX cards in parallel, which required as much PCIe slots as possible, which in turn meant high-end motherboards, which always mean putting in a modern 64bits CPU.
And as Linux is trivial to install and use in 64 bits flavor.
(On the other hand, I don't know about mining rigs running on Windows (for drivers reason, maybe ?), perhaps 32bits is still popular there ?)