in the context of V8 "x87-platform" means "x87-only for floating point" as opposed to normal ia32 platform where V8 assumes SSE2 is present. It's implemented as a separate platform port, not as a bunch of if's inside the ia32 port.
azakai didn't say it's a platform. But it is a part of the x86 instruction set that you can entirely avoid. So it makes perfect sense for support for it to be added later.