This wouldn't help at all. Kernels (micro or no) can only implement preemption with the help of hardware timers (and, if you want parallelism, hardware cores), which Webassembly cannot access. (The C library does not implement threads; it merely wraps the kernel's implementation of them.)

So at some level WebAssembly itself must provide some sort of primitive for threading, or else at best you'll get cooperative coroutines/an event loop.