runtime: manually initialize xorshift state

This ensures:

 1. The xorshift state is initialized during interp.
 2. The xorshift state gets initialized to a real random number on
    hardware that supports it at runtime.

This fixes a big binary size regression from the previous commit. It's
still not perfect: most programs increase binary size by a few bytes.
But it's not nearly as bad as before.

src/runtime/scheduler_none.go

@@ -13,6 +13,7 @@ const hasParallelism = false
 // With the "none" scheduler, init and the main function are invoked directly.
 func run() {
 	initHeap()
+	initRand()
 	initAll()
 	callMain()
 	mainExited = true