%ebx on i386 (now used for PIE). Not 100% sure if this is the best way to go,
but it works in initial tests, and the port no longer builds without it,
so let's try this for now and see how it goes, jsg@ agrees.
There's a bit more commentary in a comment in the patch (the enabled code
also aligns stack on 16 bytes which OSX IA-32 ABI requires as they use SSE
extensively, which we won't need but it seems safer to enable all the
related code together).