The Intel MultiProcessor Specification version 1.1, describing systems which used the discrete Intel 82489DX APIC or its equivalent (built into 75mhz or faster Pentium processors) includes 80486 systems, and companies such as ALR made compliant boxes. (Old enough versions of) Linux *in theory* should work on MP 1.1 compliant 486 boxes, possibly with some tweaking. Of course it's very difficult to find much in the way of user reports because by the time Linux meaningfully supported SMP at all those machines were getting to be pretty long in the tooth and they were never common.
Linux on proprietary non-SMP architectures like the Compaq SystemPro or Sequent's early 386 "mainframes" is another story, although I seem to recall there being at least some experimental SystemPro support at one point. (Whether "experimental" meant "really actually works" or not is another question.)