All of the information I've seen thus far on Meltdown and Spectre explicitly reference 64-bit platforms. What about 32-bit (specifically RHEL/CentOS)? I would assume that's also vulnerable but can someone confirm if that's the case?
Any CPU that performs speculative execution is vulnerable to Spectre, so yes, 32-bit OSs are vulnerable.
Meltdown is an issue with how Intel CPUs enforce memory protection while performing speculative execution (in short, memory protection isn't enforced until the point at which speculative execution is turned into real execution). 32-bit OSs on Intel CPUs are vulnerable, but the heavier use of swap reduces the impact somewhat (Meltdown can only read physical memory; data that's been swapped out to disk is inaccessible).